The effects of bull exposure and lasalocid on the development of replacement beef heifers

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1 The effects of bull exposure and lasalocid on the development of replacement beef heifers J. A. Small, R. P. Del Vecchio 1, W. P. McCaughey, D. R. Ward, and W. P. Sutherland Agriculture and Agri-Food Canada, Research Centre, P. O. Box 1000 A, Brandon, Manitoba, Canada R7A 5Y3 ( Received 1 December 1999, accepted 17 August Small, J. A., Del Vecchio, R. P., McCaughey, W. P., Ward, D. R. and Sutherland, W. P The effects of bull exposure and lasalocid on the development of replacement beef heifers. Can. J. Anim. Sci. 80: At weaning in the fall, crossbred heifers (n = 224), born in either the winter (January February) or spring (March April), were assigned on the basis of age, sirebreed and body weight to one of two similar winter housing facilities (with or without sterilized bulls), and to one of two foragebased (87%) diets (with or without lasalocid, 200 mg d 1 ) within each housing facility. Observations for estrus were made twice daily. Timed AI (66 h after PGF 2α ) was used to breed heifers for the first time at 14 mo of age. Plasma progesterone concentrations were used to confirm estrus/ovulation and to determine the PGF 2α response rate. Bull exposure advanced puberty in winterborn heifers, but delayed puberty in spring-born heifers (P 0.029). Similarly, timed AI pregnancy for winter-born heifers was higher with than without bull exposure (58.9 vs ± 5.3%; P = 0.017) while the opposite occurred for the spring-born group (27.1 vs ± 4.7%; P < 0.001). Bull-exposed spring-born heifers were the oldest at calving, the latest to calve, and their calves had the slowest growth and lowest weaning weight means (P < 0.027). Lasalocid did not influence puberty (P 0.273), had a small effect on body weight gain (P 0.033) that did not limit attainment of optimal body weight or condition at AI, but enhanced response rate for spring-born heifers (P = 0.075) and conception rate for winter-born heifers (P = 0.047). The efficacy of bull exposure and lasalocid is dependent upon the proximity of heifers to the attainment of puberty when the treatments are introduced; further research is required to determine the most appropriate use of either management tool for developing beef replacement heifers. Key words: Puberty, heifer development, bull exposure, ionophore, estrus, conception Small, J. A., Del Vecchio, R. P., McCaughey, W. P., Ward, D. R. et Sutherland, W. P Effets de la présence de taureaux et de l administration de lasalocide sur le développement sexuel des génisses de remplacement de type à viande. Can. J. Anim. Sci. 80: Lors du sevrage à l automne, des génisses croisées (n = 224) nées soit á l hiver (janvier février) ou au printemps (mars-avril) dernier étaient réparties par catégorie d âge et selon la race paternelle et le poids vif, entre deux installations semblables de logement d hiver, en présence ou non de taureaux stérilisés et deux régimes alimentaires à base de fourrages grossiers (87 %), avec ou sans administration de lasalocide à raison de 220 mg j 1 dans chaque installation. La détection des chaleurs se faisait deux fois par jour. Pour la première mise à la reproduction, à l âge de 14 mois, on utilisait l IA programmé, soit 66 h après traitement de PGF 2α. Les concentrations plasmatiques de progestérone servaient de base pour la confirmation de l oestrus et de l ovulation et pour établir le taux de réponse à la PGF 2α. La présence des taureaux accélérait la venue de la puberté chez les génisses d hiver, mais la retardait chez les génisses de printemps (P 0,029). Ces dernières avaient, par ailleurs, un taux de gravidité après IA plus élevé en l absence qu en présence de taureau, soit 27,1 contre 59,1 ± 4,7 % (P < 0,001), alors que l inverse s observait chez les premières : soit 58,9 contre 32,5 ± 5,3 % (P < 0,017). Les génisses de printemps exposées à la présence des taureaux étaient les plus vieilles et les dernières à vêler et leur veau avait le GMQ et le poids au sevrage les plus bas. L administration de lasalocide n avait pas d effet sur l arrivée de la puberté (P 0,273) et montrait un petit effet sur le GMQ (P 0,033), qui sans restreindre l obtention du poids ou de l état corporel optimal au moment de l insémination, augmentait le taux de réponse à la PGF 2α chez les génisses de printemps (P = 0,075) et le taux de conception chez les génisses d hiver (P = 0,0047). L efficacité de la mise en présence des taureaux et du traitement au lasalocide dépend du temps qui à ce moment aux separe les génisses de la puberté. Il reste à établir le mode d utilisation le plus approprié de ces deux outils d élevage dans la production des génisses de remplacement de type à viande. Mots clés: Puberté, croissance, maturité sexuelle des génisses, mise en présence de taureau, ionophore, oestrus, conception Establishment of pregnancy at 15 mo of age is necessary for heifers to calve as 2-yr olds early in the calving season. Heifers that calve early in their first calving season generally have higher lifetime calf production than those that calve late (Lesmeister et al. 1973; Laflamme 1993). Approximately 40% of heifers have abnormal length estrous 1 Present address: Louisiana State University, Baton Rouge, LA, USA, cycles after the first or second pubescent estrus (Del Vecchio et al. 1992) and the fertility of heifers was 21% lower when breeding occurred at the first rather than at the third estrus (Byerley et al. 1987). Therefore, management of Abbreviations: AI, artificial insemination; BE, bull exposure (bull sterilized by epididymectomy); LA, lasalocid (Bovatec ); NE, no bull exposure; NL, no lasalocid; PGF 2α, prostaglandin F 2α (Lutalyse )

2 616 CANADIAN JOURNAL OF ANIMAL SCIENCE replacement heifers has been directed towards the early attainment of puberty, which may be influenced by many environmental (social and climatic), nutritional and genetic factors (Wiltbank et al. 1966; Vandenbergh 1989). Sterilized bulls (bull exposure) and ionophore supplementation have been used to stimulate puberty in heifers, but results have been inconsistent. Housing mature sterilized bulls with heifers for a period of time prior to breeding has failed to affect age or body weight of heifers at puberty (Berardinelli et al. 1978; Roberson et al. 1987), reduced age at puberty (Roberson et al. 1991), and advanced calving date (Makarechian et al. 1985). The inclusion of ionophores, such as lasalocid and monensin, in postweaning diets has been shown to reduce age at puberty in heifers (Moseley et al. 1977; McCartor et al. 1979; Goehring et al. 1984) and bulls (Rutter et al. 1991). However, in heifers, advanced puberty did not affect pregnancy rate and the effects of feeding ionophore may have been confounded by the use of sterilized bulls to detect estrus (Mosely et al. 1977; McCartor et al. 1979). Furthermore, there is a lack of information on the effects of bull exposure or ionophores on the subsequent maternal ability of heifers. Prostaglandin analogues have been used to synchronize breeding of cycling cows and heifers and have been shown to be an economical way to increase weaning weights (Gaines et al. 1993). In Manitoba, the calving season is prolonged because a low proportion of cows and heifers become pregnant early in the breeding season. Although producers consider reproductive performance a top factor limiting the profitability of their operations, AI and synchronized breeding have not been widely adopted because of the time required for estrus detection (Small and McCaughey 1999). Therefore, incorporation of timed AI, which does not require estrus detection, into a management program for replacement heifers may be an effective way to increase early pregnancy rates. The objectives were to determine the effects of postweaning environment (presence or absence of sterilized bulls) and diet (with or without lasalocid) on the reproductive development of replacement beef heifers. The study examined the occurrence of pubescent estrous cycles, timed-ai pregnancy and conception rate, overall pregnancy rate after bulls were used for return services to complete a 45-d breeding season, postweaning rate of gain in body weight and subsequent calving, breeding and calf production ability as 2-yr olds. MATERIALS AND METHODS Treatments and Animals Two-hundred and twenty-four crossbred heifers were assigned after weaning to one of two housing environments, bull exposure (BE) and no exposure (NE), and one of two diets, with lasalocid (LA) and without lasalocid (NL), within each housing environment. Heifers were assigned on the basis of age, breed, body weight and source (purchased or resident) to one of two similar facilities and to one of two pens within each facility. Sterilized bulls were housed with the heifers in one facility Table 1. Composition of diets z for replacement beef heifers y,x Corn silage Forage Barley greenfeed Amount day 0 to day 56 (% diet DM) Amount day 56 to AI (% diet DM) Dry matter (g kg 1 ) Digestible energy (MJ kg 1 ) Protein (g kg 1 ) Neutral detergent fiber (g kg 1 ) Acid detergent fiber (g kg 1 ) Calcium (g kg 1 ) Phosphorus (g kg 1 ) Magnesium (g kg 1 ) Potassium (g kg 1 ) z Based on laboratory analysis of feed samples taken at the time of conservation. y Postweaning to turnout: wheat distillers grain (1.5 kg d 1 ) with or without lasalocid (200 mg d 1 per head) was mixed with the forage before offering the total mixed ration. Wheat distillers grain provided 13% of dry matter intake for the first 56 d and 10% from 56 d to AI at 14 mo of age. The amount of silage and greenfeed was adjusted so that there was approximately 10% of the mixed ration refused daily. x Subsequently, the pre-calving diet consisted of barley silage (48%), hay (38.5%), limestone (0.2%) and a peleted grain supplement (13.8%). Barley silage (39.3% DM) contained on a g kg 1 dry matter basis: protein 123.8, acid detergent fiber 345, neutral detergent fiber 541, P 2.7, Ca 3.9, K 19.0, Mg 1.8, S 3.6, Cl 0.8. Grass-legume hay (85.3% dry matter) contained on a g kg 1 DM basis, protein 161, acid detergent fiber 390 g, neutral detergent fiber 610, Ca 7.0, P 2.6, K 29.2, Mg 1.6, S 2.3 and Cl 4.9. The peleted supplement contained per MT crushed barley (953 kg), trace-mineral mix (28.0 kg containing Ca 155, P 155, Mg 20, K 10, I 0.2, Fe 5.0, Cu 4.0, Mn 50, Zn 10, Co 0.05, F 2 g kg 1 and vitamins A 500, D3 55, E 0.5 KIU kg 1 ), cobalt-iodized salt (15 kg), strong A, D, E (0.70 kg containing 1000 KIU A, 100 KIU D3, 0.5 KIU E), rumensin (0.76 kg containing 200 g kg 1 monensin) and MgSO 4 (2.5 kg). After calving the heifers were given barley grain (2 kg d 1 ) in addition to hay, silage and the peleted supplement. to provide the BE environment treatment, and comparable number of non-test heifers were placed in the other facility to provide the NE treatment. Heifers in the west pen in each facility received the LA treatment and the east pen received the NL treatment. The two facilities were located about 80 m apart on a hillside sloped to the south with deciduous trees providing additional shelter on the north, south, east and west. Each facility consisted of a southern exposed shed, which provided a straw-bedded rest area (4.88 m 2 and 3.72 m 2 per heifer for BE and NE, respectively) on the north end of a drylot (12.86 m 2 and m 2 per heifer for BE and NE, respectively) with bunk feeders (598 mm per heifer) at the southern end of the lot. The lot and rest areas and bunk space met or exceeded specifications for this class of livestock and type of housing facility (Whitaker 1979). The sterilized (epididymectomized) bulls (Composite or British crossbred bulls) 7 to 8 mo of age and exhibiting mounting behavior at the start of the study, were housed with the heifers at a ratio of 20 heifers to one bull. To change the social order and maintain libido, the sterilized bulls were sorted into different groups with two bulls substituted in rotation so that a different group of bulls would be introduced to heifers every 28 d. Heifers were given a total mixed ration (Table 1) of forage and a supplement of wheat distillers grain formulated

3 SMALL ET AL. HEIFERS, BULL EXPOSURE AND LASALOCID 617 according to the National Research Council (1996) to provide an average body weight gain of 0.75 kg d 1 when offered free-choice. The supplement was prepared with or without Bovatec (LaRoche Animal Health, Inc., Nutley, NJ) to provide lasalocid at either 0 or 200 mg d 1. The amount of forage offered was adjusted daily so that approximately 10% was left in the bunks prior to the next feeding. At day 56 the amount of forage was increased from 87 to 90% of the ration on a dry matter basis. For the last 28 d (day 168), with the spring-born group, green feed was replaced with chopped barley straw and steam-rolled barley because of an insufficient supply of green feed. Heifers used in this study were crosses of Continental (Simmental or Composite) and British (Red Angus or Hereford) breeds born between January and May at either the Brandon Research Centre (n = 148) or purchased from a commercial producer (n = 76). The Composite breed was 1 4 Simmental, 1 4 Charolais, 1 /16 Limousin, 7 /16 British. Body weight per day of age and age when the study started were 1.16 ± 0.01 and 1.20 ± kg and 9.1 ± 0.02 and 7.7 ± 0.05 mo for winter- and spring-born groups, respectively. Calves had been weaned in late September 1997 and the study began on 5 November Cattle were handled in accordance with the guidelines of the Canadian Council on Animal Care (1993) and the Canadian Code of Practice for the care and handling of beef cattle (Agriculture Canada 1991). Estrus Detection and Breeding Heifers were observed twice daily for 45 min at 0830 and 1500 h for signs of estrus (standing heat). Estrus/ovulation was confirmed on the basis of plasma progesterone concentrations. Twice-daily observations of estrus were recorded from the start of the study until the breeding program began, but progesterone confirmation of estrus/ovulation was discontinued for individual heifers after three confirmed estruses. Observations of estrus for spring-born heifers continued for 2 mo after the trial with winter-born heifers was completed. Estrous cycle length was calculated as the number of days between confirmed estruses. The first service for all heifers was timed AI using proven Gelbvieh semen (ABS Global, DeForest, WI). Heifers were synchronized and inseminated in two groups, so that all would receive the first service at mo of age (6 April and 25 May 1998 for winter- and spring-born groups, respectively). The timed AI program consisted of two doses of PGF 2α (5 ml Lutalyse i.e., 25 mg dinoprost tromethamine per dose; Pharmacia & Upjohn, Orangeville, ON) given at 1400 h on day 1, 1400 h on day 11 and AI at 0800 h on day 14. However, the initial dose of PGF 2α was not given to heifers observed in estrus 0 to 4 d before; or expected to be in estrus 1 to 4 d after the first injection. Fertile Gelbvieh bulls were placed on pasture with spring-born heifers from 14 to 45 d after AI. The winter-born heifers that did not conceive to AI had an opportunity for return service only when turned out with the spring-born group. On pasture, the ratio of heifers to bulls was high (70:1) because it was assumed that approximately 50% of heifers would not return to estrus and returns would not be tightly synchronized and, also, the bulls used were experienced 2 yr-olds. The herd veterinarian evaluated all bulls for breeding soundness before the breeding season. Sterilized bulls used for the BE treatment were tested for the absence of spermatozoa in ejaculates and for trichomoniasis. Real-time transrectal ultrasonography (Aloka 560, 5.0 MHz probe) was performed 42 d after AI to determine timed AI pregnancy rate (number of heifers at 42 d gestation/number time-inseminated). Transrectal palpation was used to determine overall pregnancy rate 60 d after the end of the breeding season (number pregnant/number exposed to timed AI and bulls). Conception rate was defined in terms of respondents (number of respondents pregnant to AI/number of respondents). Body Weight, Gain and Condition Score Heifers were weighed every 28 d throughout the postweaning period until the timed AI. All weights were taken before feeding. Initial and AI weights were the mean of weights taken on 2 consecutive days, otherwise single weights were taken, and the average daily rate of gain in body weight was determined for individual animals by regression analysis (SAS Institute, Inc. 1988). Body condition was scored on a scale of 1 (thin) to 9 (obese) beginning when heifers were yearlings. Following timed AI, body weights and condition scores were taken at the end of the breeding season, the end of the pasture season, midgestation, at calving, and at the start and end of the postpartum breeding season and at fall weaning. Blood Sampling and Progesterone Analysis Blood samples were collected for progesterone analysis 7 to 10 d after estrus, at the time of PGF 2α injections, and at AI. All blood samples were collected into 10-mL heparinized evacuated tubes by venipuncture of the coccygeal vein or artery. Progesterone concentrations in blood plasma were determined by an enzyme-immunoassay with a detection limit of 0.03 ng ml 1 (Del Vecchio et al. 1995). The coefficients of variation within and between assays were 7.1% (n = 8) and 10.2% (n = 20), respectively. For confirmation of estrus/ovulation and the ovarian response to PGF 2α, a progesterone concentration greater than or equal to 1.0 ng ml 1 was considered indicative of a functional corpus luteum whereas a concentration less than 1.0 ng ml 1 was considered indicative of the absence of a functional corpus luteum. Heifers with a high progesterone concentration at the time of the second dose of PGF 2α and a low progesterone concentration at AI were considered respondent. Non-respondents included asynchronous (high progesterone at AI) and non-cycling (low progesterone throughout the synchronization program) heifers. Maternal Management Breeding to Weaning Heifers were brought in from pasture in the fall (4 October 1998). Due to limited resources it was not possible to keep all bred heifers to determine the carry-over effect of the postweaning treatments on maternal performance. Therefore, only pregnant Simmental-cross heifers bred during the

4 618 CANADIAN JOURNAL OF ANIMAL SCIENCE May June breeding season were kept (22 winter-born and 55 spring-born heifers). The heifers were wintered as one group in a southern-exposed shed with an open lot and bunk feeder. Throughout gestation the heifers were fed barley silage, hay and trace-mineralized salt (Table 1). Beginning 3 wk prior to calving, all heifers were fed a pelleted supplement (2 kg d 1 ) and limestone. After calving, heifers were fed rolled barley (2 kg d 1 ) in addition to the pelleted supplement. During the calving season, heifers were housed outdoors with access to a loose housing shelter, well bedded with straw. Experienced herdspersons monitored calving activity 24 h a day during the first 8 wk of the calving season. Calving difficulty was scored either unassisted or assisted. Assistance by hand pull or fetal extractor was given if the calf was not delivered within 90 min of rupture of the fetal membranes, or if there was no progress after 60 min of severe straining. After parturition, the heifer and her calf were moved to clean indoor, straw-bedded pens for a period of 1 to 3 d after calving to ensure bonding before turning the pair out to a postcalving lot. Within 24 h of birth calves were weighed, ear-tagged, injected with vitamin E and selenium (Dystocel ), the navel disinfected, and elastrators positioned for castration of male calves. The post-calving area also had a drylot and straw-bedded loose housing shed with an area for calves to voluntarily rest separate from dams. The first breeding postpartum was timed AI on 25 May 1999 at 66.4 ± 15.6 d after calving followed by turnout and bull service for 43 d. Ultrasonography and palpation were used to determine AI pregnancy and overall pregnancy rates as in the first year. Heifer/calf pairs grazed grass-legume pastures until weaning (5 October 1999). No creep feed or growth implants were given to calves. The winter-born group calves were progeny of the Gelbvieh bulls whereas 47.3% of the spring-born group calf crop was derived from the AI sire and the remainder from the Gelbvieh bulls. The calf-crop was 45.4% and 43.6% female for winter- and spring-born groups, respectively. All calves born were weaned except for two calves from the BE + NL group because one broke a leg and a cow/calf pair died from dystocia. Statistical Analysis Statistical analyses of the data were performed using the SAS p.c. v 6.12 (SAS Institute, Inc. 1988). General Linear Models (GLM) procedures with Type III sums of squares were used for body weight (repeated measures ANOVA), the rate of gain and calving date. Categorical models (CAT- MOD) procedures were used to test differences in proportions (pubescent or three estruses, long and short cycles, PGF 2α respondent, conception and pregnancy. The main effects in the model for winter- and spring-born heifers were bull-exposure (BE), lasalocid (LA), and the interaction between bull-exposure and lasalocid. The error terms were animals within diet within environment for main effects and animals within diet within environment within day for subplot (time) effects of time and interactions between time and the main effects of BE, LA and BE LA. RESULTS Puberty and Pubescent Estrous Cycles Bull-exposure, but not lasalocid, had significant (P 0.029) effects on attainment of puberty. Advanced occurrence of pubescent estrus in bull-exposed winter-born heifers was evident within 14 d and persisted throughout the study (Fig. 1a). Delayed occurrence of pubescent estrus for bullexposed spring-born heifers was evident within 70 d, although within the first 6 wk of the trial, the trend appeared to favor BE (Fig. 1b). The effects of bull-exposure on the occurrence of first estrus diminished at around 12 mo of age, which was approximately day 56 and day 112 of the study for winter- and spring-born groups, respectively. Postweaning treatments did not (P 0.132) influence the proportion of winter- and spring-born heifers that had one, two or three estruses with ovulation before initiation of the synchronization program. However, fewer long first estrous cycles occurred in bull-exposed groups (Table 2). Estrus detection rate was not (P 0.181) influenced by postweaning treatments. The number of estruses observed for heifers with and without bull exposure was 428 and 473, respectively, of which 19 and 26 ± 3% were confirmed false (progesterone low after estrus). Estruses detected for heifers with and without lasalocid were 458 and 453 of which 24 and 22 ± 3% of confirmed estruses were false. Estruses detected for winter- and spring-born groups were 410 and 491 of which 20 ± 2 and 25.4 ± 4% were false. Response and Pregnancy Rates The winter- and spring-born groups had similar response to PGF 2α (73.2%), timed AI pregnancy (44.1%) and conception (57.8%) rates; however, the two groups showed different effects of bull-exposure and lasalocid (BE LA interaction P 0.323) on the success of the timed-ai program (Table 3). Overall pregnancy rates (91.3%) were not different (P 0.200) among groups. For winter-born heifers, response and AI pregnancy rates were approximately 30% higher with, than without bull exposure. However, the conception rate was approximately 30% higher with, than without lasalocid. For spring-born heifers, response rates were not influenced by bull exposure but tended (P = 0.075) to be highest for the LA group, while timed-ai pregnancy and conception were approximately 35% lower for BE than NE groups. Body Weight, Gain and Condition Score Mean body weights for winter- and spring-born heifers in bull exposure and lasalocid treatment groups are shown in Fig 2. There were small, inconsistent differences in body weight that occurred within the first 28 d of the study which caused Time BE LA interaction in winter-born (P = 0.028) and spring- born (P = 0.022) groups. For the winter-born group the interaction occurred because body weight gain during the first 28 d of the trial was lowest for lasalocid-fed heifers especially with bull exposure. For the spring-born group the interaction occurred because body weight gain during the first 28 d of the trial was highest for lasalocid-fed heifers, especially those with bull-exposure.

5 SMALL ET AL. HEIFERS, BULL EXPOSURE AND LASALOCID 619 Table 2. The proportion of spring- and winter-born beef heifers that exhibited at least two or three periods of estrus with ovulation between weaning and first service at 14 mo of age and the proportion of first and second estrous cycles that were of normal, short, or long duration Birth group Winter Spring Number of heifers One estrus/ovulation (%) Two estrus/ovulations (%) Normal 18- to 24-d cycle (%) Short cycle (%) Long cycle (%) z Mean (± SEM) first cycle length (d) z 25.8 ± ± 1.5 Three estrus/ovulations (%) Normal 18- to 24-d cycle (%) Short cycle (%) Long cycle (%) Mean (± SEM) second cycle length (d) 18.6 ± ± 1.0 z Mean first cycle length was shorter and the proportion of long cycles lower for spring-born heifers with than without bull exposure (18.8 vs ± 2.1 d; P = and 10.2 vs 28.6 ± 2.8%; P = 0.016). For winter-born heifers first cycle length and the proportion of long first cycles did not differ between BE and NE groups (24.8 vs ± 3.0 d; P = and 22.2 vs ± 3.6%; P = 0.323). Fig. 1. The effects of bull exposure on the cumulative proportion of (a) winter (January February) and (b) spring (March April) born heifers that exhibited one (circles), two (diamonds) or three (squares) confirmed estruses from the introduction of treatments on 5 November, after weaning in late September, to timed insemination at 14 mo of age. The dashed line indicates yearling age for the oldest heifers. These initial differences in body weight influenced the overall rate of body weight gain shown in Table 3. Weight gain for winter-born heifers was approximately 70 g d 1 lower (P = 0.015) for LA than NL treatments, which did not influence body weight or condition score at AI (395 vs. 400 ± 3.2 kg; P = and 6.1 vs. 6.3 ± 0.08; P = 0.221) with or without an adjustment for initial body weight. Weight gain for spring-born heifers averaged 50 g d 1 higher (P = 0.033) for LA than NL, which resulted in slightly greater body weight with no effect on body condition at AI (437 vs. 426 ± 2.7 kg; P = and 6.2 vs. 6.2 ± 0.05; P = 0.771). Maternal Performance The carry-over effect of the post-weaning treatments on maternal performance is shown in Table 4. For the springborn group the time from AI to calving was 283 ± 1.6 d for heifers that conceived to AI and 312 ± 1.4 d for heifers bred by the bulls on return services. Bull-exposed spring-born heifers were the oldest at calving (P = 0.017), the latest to calve (P = 0.011), and their calves had the slowest growth (P = 0.027) and lowest weaning weight (P = 0.011). The

6 620 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 3. The effects of bull exposure and lasalocid (mg d 1 ) z on postweaning body weight gain, response to PGF y 2α and timed AI pregnancy at 15 mo of age of crossbred beef heifers born in winter or spring Bull exposure (BE) Lasalocid (LA) Probability No Yes SE BE LA BE LA Winter (January February) born heifers Number of heifers Body weight gain (kg d 1 ) Response rate (%) Timed-AI pregnancy rate (%) Conception rate (%) Spring (March April) born heifers Number of heifers Body weight gain (kg d 1 ) Response rate (%) Timed-AI pregnancy rate (%) < Conception rate (%) < z Sterilized (epididymectomized) bulls and lasalocid (mg d 1 ) introduced 5 November after weaning late September. y Plasma progesterone (1 ng ml 1 at the time of PGF 2α (25 mg i.m.) and < 1 ng ml 1 at the time of AI (66 h later). heifers given lasalocid remained as the heaviest group at the end of the pasture season (522 and 537 ± 5.9 kg, for NL and LA treatments, respectively, P = 0.086) and at calving. Body condition score was similar (P 0.336) among groups at the end of the pasture season (6.0 ± 0.05) and at calving (4.7 ± 0.08). Calving ease was not influenced by postweaning treatments, but was greater for heifers that conceived to timed-ai than bull service (73.0 vs. 48.3% unassisted; P = 0.061). Postpartum timed-ai pregnancy rate (51.8%; n = 54) did not differ (P 0.540) among treatments. However, overall pregnancy rates showed significant interaction between BE and LA treatments (P = 0.033) because pregnancy rates for heifers given either bull exposure or lasalocid (89.7%; n = 29) were higher than for heifers given no treatment or the treatments in combination (64.0%; n = 25). For the winter-born group, the interval from the start of the bull-breeding period to calving was shorter for either BE or LA treatments than for the combined treatments of either NE + NL or BE + LA [294 (n = 16) vs. 310 (n = 13) ± 8.7 d; BE LA interaction; P = 0.018]. Similarly, calf weaning weights were higher for either BE or LA treatments than for the combined treatments (235 vs. 215 ± 15.9 kg, BE LA Fig. 2. Mean body weights for winter- (January February) and spring- (March April) born heifers housed with (BE) or without (NE) sterilized bulls and given a forage-based ration with (LA) or without (NL) lasalocid from the introduction of treatments on 5 November, after weaning in late September, to timed insemination at 14 mo of age. (Winter-born time BE LA; P = 0.028; spring-born time BE LA; P = ) interaction; P = 0.081). There was no influence of treatment on the mean body weight and condition score of heifers at the end of the pasture season (507 ± 8.5 kg and 6.0 ± 0.07) and at calving (517 ± 8.4 kg and 4.8 ± 0.18), respectively. Calving ease (68.2% unassisted), postpartum timed AI pregnancy (50.0%) and overall pregnancy (86.4%) did not differ among treatment groups (P 0.336). DISCUSSION Effects of Bull Exposure Puberty was advanced in winter-born heifers, but delayed in spring-born heifers when bull exposure treatment was introduced approximately 1 mo after weaning in the fall. The winter-born heifers were pubescent and the spring-born heifers prepubescent at the start of the study as evidenced by the cumulative occurrence of estrus. Within the first 21 d of the study, 30% of winter-born and only 3% of the springborn heifers attained puberty. Within 90 d, 80% of winterborn heifers had attained puberty but it took 123 d for 80% of spring-born heifers to reach puberty. Therefore, the winter-born heifers were close to the onset of estrous cycles when the trial started and bull exposure stimulated an increase in the occurrence of pubescent estrus and enhanced

7 SMALL ET AL. HEIFERS, BULL EXPOSURE AND LASALOCID 621 Table 4. The carry-over effects of postweaning bull exposure and lasalocid (mg d 1 ) treatment of prepubescent beef heifers on subsequent maternal and rebreeding performance Bull-exposure (BE) Lasalocid (LA) Probability No Yes SE BE LA BE LA Number of heifers Age at calving (d) Calving date (d) z Calving weight (kg) y Calf weight gain (kg d 1 ) x Calf weaning weight (kg) y Timed AI pregnancy (%) Pregnancy (%) w z The number of days from 19 February, which is 15 d before the predicted 285-d AI calving date of 6 March, y Adjusted for post-weaning body weight of the dam. x Adjusted for birth weight. w Includes timed AI and bull service of return heats from 2 to 45 d after AI. the efficacy of timed AI. In contrast, the spring-born heifers were far from the onset of estrous cycles at the start of the study, and bull exposure delayed puberty and reduced the efficacy of timed-ai. Our finding that pubescent and prepubescent heifers respond differently to bull exposure may explain some of the inconsistencies in the literature. Both Makarechian et al. (1985) and Roberson et al. (1991), using yearling crossbred heifers, concluded that bull exposure increased the proportion of heifers that conceived early in the breeding season, but had no effect on pregnancy rate. However, Roberson et al. (1987), using heifers 8 mo of age, concluded that bull exposure had no effect on attainment of puberty and showed that the proportion of heifers that were pubescent at 14 mo of age was approximately 7% lower with, than without bull exposure. Therefore, there appears to be a time during the reproductive development of heifers when bull exposure will be either detrimental or beneficial to the establishment of normal length estrous cycles and establishment of pregnancy early in the breeding season. Gonzalez-Padilla et al. (1975b) also showed that prepubescent and pubescent heifers respond differently to treatments intended to stimulate the onset of estrous cycles. Application of estradiol and progesterone treatments that mimicked changes in estradiol and progesterone concentrations in blood normally seen at puberty were able to induce puberty in pubescent but not prepubescent heifers. Puberty is the first spontaneous estrus/ovulation that occurs following a process where the endocrine function of the hypothalamus, pituitary, ovaries and uterus become synchronized (Dodson et al. 1988; Del Vecchio et al. 1992). It has been well established in cattle and sheep that puberty or postpartum resumption of estrous cycles is dependent upon synchronization of gonadotrophin-releasing hormone, luteinizing hormone and estradiol secretion among the hypothalamus, pituitary and ovaries (Gonzalez-Padilla et al. 1975a; Dodson et al. 1988). The occurrence of standing heat and elevated progesterone a week later were evidence of this synchrony to cause a follicle to ovulate and luteinize. The high proportion of abnormal length first and second estrous cycles in pubescent heifers in this and other studies (Byerley et al. 1987; Del Vecchio 1992) are evidence of asynchrony during reproductive development. The occurrence of long estrous cycles following the first pubescent estrus was less frequent for bull-exposed groups, suggesting that bull exposure altered ovarian responsiveness to uterine hormones, perhaps either prostaglandins or oxytocin. The mechanism by which the presence of bulls influences estrus and ovulation is not completely understood although it has been proposed to be related to the impact of pheromones and/or stress on the hypothalamo-pituitarygonadal axis. Pheromones have been implicated by the fact that oronasal treatment of 10-mo-old heifers with bull urine increased the proportion of heifers that were pubescent as yearlings and advanced calving (Izard and Vandenberg 1982). It has been postulated that pheromones have a priming effect on ovarian function because of enhanced luteinizing hormone secretion as has been shown in mice (Bronson and Desjardins 1974) and sheep (Chesworth and Tait 1974). It has been suggested that the presence of a male represents a stress factor to the susceptible female, stimulating an adrenal gland response (Edgar and Bilkey 1963). As reviewed by Moberg (1976), adrenal progestins and glucocorticoids are involved in the effects of stress on reproduction. Bull exposure may have caused an initial transient rise in progesterone, which caused a number of older heifers, and a few young heifers to express estrus shortly after the study began. However, if this were the case, the transient rise in the young heifers was apparently detrimental to normal reproductive development. Administration of progestins in prepubescent heifers has been shown to affect the frequency of the pulses of luteinizing hormone in blood in a dose-dependent manner, with low doses stimulating an increase, while high doses suppress luteinizing hormone release. Progestins advanced puberty only in heifers in which estradiol negative feedback has begun to decline (Anderson et al. 1996). A transient rise in blood progesterone concentrations has also been reported for heifers (Gonzalez-Padilla et al. 1975a) and postpartum cows (Stevenson and Britt 1979) prior to the establishment of fertile estrous cycles. However, early exposure to elevat-

8 622 CANADIAN JOURNAL OF ANIMAL SCIENCE ed levels of progesterone may have a carry-over effect on pregnancy establishment since recent evidence in postpartum cows indicates that ovulation too soon after calving (before 21 d) is actually detrimental to fertility at 60 d after calving (Smith and Wallace 1998). Effects of Lasalocid In this study, lasalocid had inconsistent effects on weight gain, no effect on puberty and appeared to have influenced luteal function. In general, ionophores are expected to improve the performance of cattle fed forage rations by altering microbial metabolism in the rumen to favor propionate at the expense of acetate, which is used more efficiently for body weight gain (Russell and Strobel 1989; Spears 1990). However, ionophores have also been shown to increase the digestion of other nutrients (e.g., starch, nitrogen and minerals) in the lower gastro-intestinal tract (Spears 1990). Body weight gain was lower for winter- and higher for spring-born heifers given lasalocid. The small difference in average daily weight gain did not affect body weight or condition at breeding. As shown in Fig. 3, the effect on weight gain was most pronounced after day 56 when the forage portion of the diet was increased from 87 to 90% of dry matter intake, which would have increased the daily intake of fiber by about 10 g. For the spring-born group the small increase in the rate of body weight gain likely reflected an increase in the efficiency of feed utilization for growth. It is difficult to explain the effect of lasalocid on weight gain for the winter-born group. Although feed was not restricted, there may have been differences between winter- and spring-born heifers with regard to voluntary feed intake that could not be measured in this study. The winter-born group achieved puberty early in the study, and the higher estrus activity may have suppressed feed intake. In both groups the effect of lasalocid was relatively short-lived which suggests, as in dairy cows on high forage rations (Weiss and Amiet 1990), that the rumen adapted to the ionophore. Others have also reported inconsistent effects of lasalocid on heifer growth. Moseley et al. (1977) reported no effect of lasalocid on growth rate or heart girth, but did observe an increase in frame size. Steen et al. (1992) reported that lasalocid had no effect on growth rate, but did increase heart girth and fat and muscle depth at the 13th rib. However, Steen et al. (1992) also found that a 10% increase in undegradable protein (from 32 to 42% of protein in concentrate) had the same effect as lasalocid on growth and body composition, which did not occur when heifers were given both lasalocid and undegradable protein. These data suggest that lasalocid may alter microbial metabolism in a way that influences changes in body composition. In the present study, the winter- and spring-born heifers were at different stages of sexual maturity (pubescent vs. prepubescent, respectively) and hence may have been partitioning nutrients towards fat and lean differently. This may explain the different response of prepubescent and pubescent heifers to lasalocid. In trials in which heifers gained from 0.4 to 0.6 kg d 1, the addition of monensin has decreased age at puberty with no effect on pregnancy rate (Mosely et al. 1977; McCartor et al. 1979). However, lasalocid has been shown to advance puberty in low-gaining (0.34 kg d 1 ) heifers but not in higher-gaining (0.57 kg d 1 ) heifers (Goehring et al. 1984). In the present study, the weight gain exceeded 0.40 kg d 1 for all but one heifer in the winter group that conceived to AI to a maximum of 0.95 kg d 1 for winter-born groups and 1.09 kg d 1 for spring-born groups. Therefore, in agreement with Goehring et al. (1984) lasalocid did not advance puberty in heifers that achieved a moderate rate of body weight gain on forage-based rations. Enhanced luteal function may explain higher response and conception rates in lasalocid-fed groups. However, there is little evidence to support this finding. Monensin has been shown to enhance pituitary responsiveness to gonadotrophin-releasing hormone (Bushmich et al. 1980) and the positive feedback action of estradiol (Randel et al. 1982) and increase luteal tissue and progesterone concentrations in luteal tissue (Bushmich et al. 1980). Lasalocid has been shown to enhance pituitary responsiveness to gonadotrophin-releasing hormone in prepubescent bulls; however, this response was observed within 7 d of lasalocid treatment and persisted throughout the pubescent and postpubescent periods (Rutter et al. 1991). Lasalocid differs from monensin with regard to affinity for cations. Lasalocid has affinity for divalent cations and equal affinity for the monovalent cations, whereas monensin does not bind divalent cations and has a stronger affinity for sodium than potassium (Spears 1990). In vitro, lasaslocid has been shown to enhance basal progesterone production only in the presence of calcium, as well as inhibit agonist stimulated progesterone production by granulosa cells (Zobell et al. 1987). These effects of lasalocid on steroidogenesis may be explained by the calcium-carrier function of lasalocid. However, monensin but not laslaocid has been shown to influence luteinizing hormone or other measures of metabolism (e.g., concentrations of potassium, magnesium, free fatty acids and insulin in blood plasma) independently of an effect on digestion (Armstrong and Spears 1988). Therefore, it may not be appropriate to extrapolate data derived from studies using monensin to lasalocid and generalize on the effect of ionophores on heifer reproductive development as the literature has tended to do (Sprott et al. 1988; Rutter et al. 1991). Puberty and Maternal Performance Relationship This study has shown negative effects of exposing prepubescent heifers to bulls on maternal performance as 2-yr olds. A greater proportion of spring-born heifers that were not exposed to bulls conceived to timed AI, and in agreement with others (Lesmiester et al. 1973; Laflamme 1993), pregnancy establishment early in the breeding season translated to early calving and heavier weaning weights. This relationship between conception and maternal performance began with the occurrence of estrus/ovulation between 10 and 12 mo of age. Puberty at this age would be expected to allow for heifers to be bred for the first time on the 3rd estrus. In contrast to the results of Byerley et al. (1987) there was no relationship between AI pregnancy and the propor-

9 SMALL ET AL. HEIFERS, BULL EXPOSURE AND LASALOCID 623 tion of heifers that experienced two or three estrous cycles. However, the use of PGF 2α to time AI may have facilitated the final stages of sexual maturation that would normally have occurred with a second or third pubescent estrus. This study showed no benefit of feeding lasalocid throughout the postweaning period with regard to puberty and maternal performance. However, the apparent enhancement of luteal function indicates that lasalocid may have been more useful if strategically fed around the time of synchronization. When considering these results, it is important to note that the heifers in this study maintained moderate rates of body weight gain on a good quality forage ration. Therefore, these results do not preclude that feeding lasalocid may benefit reproductive development of heifers when forage quality limits growth rate. Postpartum breeding performance of spring-born heifers, rebreeding performance (6 wk after AI) and calf average daily weight gain (milk production) of winter-born heifers were influenced by BE LA interaction, indicating that the effects of bull exposure and lasalocid treatments were not additive. However, a larger number of heifers would be required to substantiate this and to fully evaluate whether or not this effect is related to the composition of weight gain as a result of the time BE LA effects on body weight during the postweaning period. CONCLUSIONS In this study, puberty was advanced when sterilized bulls were introduced to heifers that were already close to puberty (pubescent) and delayed when introduced to heifers that were far from puberty (prepubescent). Pubescent (winterborn) heifers exposed to bulls and prepubescent (springborn) heifers not exposed to bulls had the greatest proportion of heifers that exhibited estrus between 10 and 12 mo of age and also had the highest timed AI pregnancy rates. Pregnancy establishment by timed AI at the beginning of the breeding season translated to early calving and heavier calves at weaning. Lasalocid slightly reduced the growth rate of pubescent heifers and slightly enhanced the growth rate of prepubescent heifers, although there was no marked effect on body weight or body condition at breeding. Although there was no effect of lasalocid on puberty, response rate and conception in respondents were enhanced suggesting that the use of lasalocid for heifers given a goodquality forage ration may be advantageous if strategically fed around the time of synchronized breeding. It is concluded that the efficacy of bull exposure and lasalocid is dependent upon how close the heifers are to the attainment of puberty when the treatments are introduced. Further research is required to determine the appropriate use of either bull exposure or lasalocid for developing beef replacement heifers. ACKNOWLEDGMENTS The authors thank D. Sykes (Senior Herdsperson), R. Kristjansson (Junior Herdsperson) and all Beef Program staff for their assistance in conducting this research. D. Lischka (ABS Canada, St Jacobs) and C. Ross (ABS Canada Ltd., Western Region) are thanked for their technical assistance with semen handling and AI. This research was funded by the Agriculture and Agri-Food Matching Investment Initiative and Hoffman LaRoche (Bovatec ), Ayerst Veterinary Laboratories (Factrel ), Pharmacia & Upjohn (Lutalyse ) and American Breeder Service (ABS) Canada (semen). Agriculture Canada Recommended code of practice for the care and handling of farm animals: Beef cattle. Agriculture Canada, Ottawa, ON. Publ. no. 1870/E. Anderson, L. H., McDowell, C. M. and Day, M. L Progestin-induced puberty and secretion of luteinizing hormone in heifers. Biol. Reprod. 54: Armstrong, J. C. and Spears, J. W Intravenous administration of ionophores in ruminants: Effects on metabolism independent of the rumen. J. Anim. Sci. 66: Berardinelli, J. G., Fogwell, R. G. and Inskeep, E. K Effect of electrical stimulation or presence of a bull on puberty in beef heifers. Theriogenology 9: Bronson, F. H. and Desjardins, C Circulating concentrations of FSH, LH, estradiol and progesterone associated with acute, male-induced puberty in female mice. Endocrinology 94: Bushmich, S. L., Randel, R. D., McCartor, M. M. and Carroll, L. H Effect of dietary monensin on ovarian response following gonadotrophin treatment in prepuberal heifers. J. Anim. Sci. 51: Byerley, D. J., Staigmiller, R. B., Berardinelli, J. G. and Short, R. E Pregnancy rates of beef heifers bred either on pubertal or third estrus. J. Anim. Sci. 65: Canadian Council on Animal Care Guide to the care and use of experimental animals. Volume 1. 2nd ed. CCAC, Ottawa, ON. Chesworth, J. M. and Tait, A A note on the effect of the presence of rams upon the amount of luteinizing hormone in the blood of ewes. Anim. Prod. 19: Del Vecchio, R. P., Neuendorf, D. A., Stahringer, R.C. and Randel, R. D Concentrations of 13,14-di-hydro-15-ketoprostaglandin F 2 -alpha, estradiol 17-β and progesterone during the prepubertal period in heifers. Theriogenology 38: Del Vecchio, R. P., Sutherland, W. D. and Connor, L. M A solid phase enzyme-immunoassay for the determination of progesterone in bovine, porcine and ovine plasma. Can. J. Anim. Sci. 75: Dodson, S. E., McLeod, B. J., Haresign, W., Peters, A. R. and Lamming, G. E Endocrine changes from birth to puberty in the heifer. J. Reprod. Fertil. 82: Edgar, D. G. and Bilkey, D. A The influence of rams on the onset of the breeding season of ewes. Proc. N.Z. Soc. Anim. Prod. 23: Gaines, J. D., Galland, J., Schaefer, D., Nusbaum, R. and Peschel, D The economic effect of estrus synchronization in beef heifers on average weaning weight of calves. Theriogenology 39: Goehring, T. B., Corah, L. R. and Riley, J. G Effect of lasalocid on growth and sexual development of heifers fed at two rates of growth following weaning. J. Anim. Sci. 59 (Suppl. 1): (Abstr.). Gonzalez-Padilla, E., Ruiz, R., LeFever, D., Denham, A. and Wiltbank, J. N. 1975b. Puberty in beef heifers. III. Induction of fertile estrus. J. Anim. Sci. 40: Gonzalez-Padilla, E., Wiltbank, J. N. and Niswender, G. D. 1975a. Puberty in beef heifers. I. The interrelationship between pituitary, hypothalamic and ovarian hormones. J. Anim. Sci. 40: