Genotype X Environmental Interactions on Reproductive Traits

Transcription

1 Genotype X Environmental Interactions on Reproductive Traits of Bovine Females. II. Postpartum Reproduction as Influenced Genotype, Dietary Regimen, Level of Milk Production and Parity by P. J. Hansen, D. H. Baik, J. J. Rutledge and E. R. Hauser J ANIM SCI 1982, 55: The online version of this article, along with updated information and services, is located on the World Wide Web at: Downloaded from jas.fass.org at University of Florida on June 1, 211

2 GENOTYPE ENVIRONMENTAL INTERACTIONS ON REPRODUCTIVE TRAITS OF BOVINE FEMALES. I1. POSTPARTUM REPRODUCTION AS INFLUENCED BY GENOTYPE, DIETARY REGIMEN, LEVEL OF MILK PRODUCTION AND PARITY 1'2,3 P. J. Hansen 4, D. H. Baik s, J. J. Rudedge and E. R. Hauser University of Wisconsin 6, Madison 5376 Summary The interaction between dietary regimen and breed or breed of sire on postpartum reproduction was examined in two experiments. Exp. 1 was a 2 2 factorial involving two breeds (Hereford and Holstein) and two dietary regimens, high (H) and low (L). In Exp. 2, females out of Holstein dams and sired by bulls of four breeds, Angus (AF), Hereford (HF), Simmental (SF) and Chianina (CF) were used in a 4 2 factorial with two dietary regimens. Postpartum traits were observed for three consecutive postpartum periods (PP1, PP2, PP3), beginning after first calving. In Exp. 1, Holsteins tended to have longer intervals to first estrus (IE) and had longer intervals to conception (IC, P.25) than Herefords in PP1 but not in PP2 or! Meat and Animal Science Paper No Research supported by the College of Agricultural and Life Sciences, Univ. of Wisconsin, Madison and Science and Education Administration, USDA. These experiments were contributions to North Central Regional Coop. Proj. NC-1, "The Improvement of Beef Cattle Through Breeding Methods." 2 Semen was provided by the following organiations: American Breeders Service, DeForest, WI; American Int. Charolais Assoc., Houston, TX; Carnation Genetics, Hughson, CA; Curtiss Breeding Service, Cary, IL; East Central Breeders Coop., Waupun, WI; Midwest Breeders Coop., Shawano, WI; Noba, Inc., Tiffin, OH and Tri-State Breeders Coop., Westby, Wl. athe authors thank E. Hoch, P. Bringle and J. Kane for their excellent technical assistance; J. Grass, who collected some of these data, and S. Kading, W. Winkler, M. Connors, C. Keuler and J. Busby for their assistance in the preparation of this manuscript. 4 Trainee of the Endocrinology - Reproductive Physiology Program. s Present Address: Dept. of Anim. Sci., College of Agriculture, Jeonbak National Univ., Jeonju 52, Korea, 6 Dept. of Meat and Anim. Sci. PP3. Cows fed H diets had significantly shorter IE and IC in PP1 and PP2, but not in PP3. Interval to conception was not recorded in PP3. There were breed x diet interactions for IE in PP1 (P.6) and PP2 (P.1) as differences in IE between L and H diets were more pronounced for the Holsteins than for the Herefords. Though not significant, Herefords fed L diets tended to have shorter intervals between estrus and conception than other groups in PP1 and PP2. In Exp. 2, breed of sire did not significantly affect any postpartum reproductive traits. Cows fed the H diet had shorter (P.1) intervals to first estrus in PP1 and shorter (P.5) intervals to first ovulation (I) in PP2 than L-fed cows. Interval to first ovulation was also shorter for H-fed cows in PP1, but the differences were not significant. The differences in IE between L and H diets tended to be greater for AF and HF cows than for SF and CF animals for PP1, but the interaction was not significant. The HF cows fed L diets had longer IE in PP2 than HF cows fed H diets, hut the other breed-of-sire groups were not influenced by dietary regimen, resulting in a breed of sire x diet interaction (P.1). Multiple regression analysis indicated that cows fed L diets that produced more fatcorrected milk and consumed less total digestible nutrients (TDN) from to 56 d postpartum had longer IE during PP1. Fatcorrected milk production, TDN intake, weight at calving and change in body weight from to 56 d postpartum were not significantly related to IE for cows receiving H diets during PP1. Data from PP2 and PP3 were pooled across diet groups for multiple regression analysis. None of the factors included in the model were significantly related to IE. We concluded that the influence of diet on postpartum reproductive function interacted with breed and parity and that the influence 1458 JOURNAL OF ANIMAL SCIENCE, Vol. 55, No. 6, 1982 Downloaded from jas.fass.org at University of Florida on June 1, 211

3 FACTORS AFFECTING POSTPARTUM REPRODUCTION IN COWS 1459 of level of milk production on IE varied with dietary level and parity. Because diet influenced reproduction more in Holsteins than Herefords, the use of dairy breeds in beef production may necessitate diets higher in energy to achieve maximum reproductive performance. (Key Words: Postpartum Cow, Nutrition, Breed, Milk Production.) Introduction Calving interval is determined by interval from parturition to first estrus, interval from first estrus to conception and gestation length. Variation in the interval to first estrus accounted for 63.8% of the variation in the interval from parturition to conception (Wiltbank, 1955) and only about 1% of the variation in calving interval was due to gestation length (Touchberry et al., 1959). Breed differences were noted in duration of the postpartum anestrus (Foote et al., 196; Rakha et al., 1971; Holness et al., 1978) and in gestation length (Preston and Willis, 197). Cows fed low energy diets had longer intervals to first estrus (Wiltbank et al., 1962, 1964; Dunn et al., 1969), longer gestation periods (Kress et al., 1971a) and lower fertility (Wiltbank et al., 1964; Folman et al., 1973) than cows fed diets higher in energy. Intervals to first estrus were longer for cows with high milk production than for low producers (Menge et al., 1962; Saiduddin et al., 1968). Primiparous cows had longer intervals to estrus (Tervit et al., 1977) and conception (de Kruif, 1975) than multiparous cows. Fertility was lower when cows were bred soon after calving (Shannon et al., 1952; Whitmore et al., 1974) compared with those bred later. Postpartum reproductive traits have been reported to be influenced by genotype x nutrition interactions (Dunn et al., 1969; Kropp et al., 1973; Whitmore et al., 1974; Bellows and Short, 1978). Our objectives were to determine interactions between breed or breed of sire and energy density of diet on postpartum reproductive traits and to determine the influence of body weight, total digestible nutrient (TDN) intake, weight gain and fat-corrected milk production on the reproductive function of postpartum cows on two nutritional regimens. The crossbred groups were chosen to reflect types of cattle that have potential use in areas where dairy cattle make up a siable portion of the bovine population and where a terminal sire system of crossbreeding for beef production might be implemented. Materials and Methods Exp. 1 was a 2 x 2 factorial, with two breeds (Holstein and Hereford) and two dietary regimens. Data were obtained from one member of each set of 31 pairs (15 Holstein and 16 Hereford) of identical or fraternal twins. At 21 d of age, the heifers were started on one of two diets of differing TDN content, High (H) and Low (L). Diet changes were made at 365 and 449 d of age to adjust for changes in the animals' nutrient requirements. Total digestible nutrient percentages of the diets and feeding management were detailed by Grass et al. (1982). The TDN intake/28-d interval after calving was estimated as individual feed consumption per 28 d x TDN percentage of the diet. Heifers were artificially inseminated at their first estrus after 15 mo of age and at all subsequent estrous periods until conception. After the first (PP1), second (PP2) but not third calvings (PP3), animals were bred at their first estrus and all subsequent estrous periods until conception. Hol- steins were bred with semen from Polled Hereford bulls and Herefords were bred with semen from Holstein bulls. The cows were palpated per rectum at weekly intervals beginning at first estrus after calving for detection of corpora lutea. All cows were suckled for 224 d postcalving. Calves were allowed with cows at all times except at feeding times and on the days that milk production was estimated. Milk production of the cows was estimated weekly by a method described by Kress et al. (1971b). Milk production levels were adjusted to 4% fat content by the equation: fat-corrected milk =.4 (milk production) + 15 (butterfat content). Exp. 2 was a 4 X 2 factorial, with four breeds of sire and two dietary levels. The heifers were out of Holstein dams and sired by Hereford (HF, n=15), Angus (AF, n=14)i Simmental (SF, n=16) and Chianina (CF, n=13) bulls. The sire breeds were chosen to provide two small (HF and AF) and two large crossbreds (SF and CF), with high (AF and SF) and low milk production (HF and CF) in each sie category. Downloaded from jas.fass.org at University of Florida on June 1, 211

4 146 HANSEN ET AL. Procedures were the same as in Exp. 1, with the following exceptions. The experimental diets were started when the heifers were 168 d of age. Dietary changes were made at 38 and 42 d of age. Estrus was determined as before, but vasectomied bulls were present in each lot. Monogamous matings were made by inseminating each female at her pubertal estrus and all subsequent estrous periods until conception with semen from one of 58 Jersey bulls. After each calving, animals were bred with semen from one of 58 Charolais bulls at all estrous periods until conception. The cows were palpated weekly beginning at d 14 postpartum for detection of ovulation and pregnancy. When a corpus luteum was detected before an observed estrus, day of ovulation was defined as 21 d before the subsequent estrus. In the event that the estrus after a quiet ovulation was not observed, first ovulation was presumed to have occurred 1 d before detection of the first corpus luteum. Statistical Analysis. The effects of breed or breed of sire and dietary level on gestation period and intervals from parturition to first ovulation, from parturition to first estrus and from parturition to conception were analyed by least-squares analysis of variance. The model included effects of season of calving (winter = October to March, and summer = April to September). Reproductive data were initially analyed with all interactions included, then reanalyed without partitioning the sum of squares for some nonsignificant interactions (breed or breed-of-sire season and breed or breed-of-sire diet x season). Nonreproductive traits were analyed by least-squares analysis of variance omitting the effect of season of calving. When breedof-sire effects were significant, mean separation was carried out by Duncan's new multiple range test. Breed or breed-of-sire effects on service interval (interval between first estrus and conception) were tested by pooling across dietary regimen and applying the Kruskal- Wallis test. Diet effects were tested by pooling across breeding groups and applying Wilcoxon's two-sample test. Because these tests do not allow the testing of interactions, breed or breed-of-sire effects were also tested within diet groups using the Kruskal-Wallis test. Spearman's correlation coefficients were calcu- lated to determine the relationship between interval to first estrus and service interval. The statistics described in this paragraph are detailed by Steel and Torrie (196). In Exp. 2 the following multiple regression model was used to partition the variance of interval to first estrus: y -- i + btbw + b2abw + b3tdn + b4fcm + bscdi + e, where i = the y intercept, bl to bs -- net partial regression coefficients, BW = body weight at calving, ABW = change in body weight from to 56 d postpartum, TDN = TDN intake from to 56 d postpartum, FCM = fat-corrected milk production from to 56 d postpartum, CDI = calving date index where = July 2 and 182 = January 1 and December 31 and e = error. Standard partial regression coefficients were calculated from the regression analyses. Results Exp. 1. The effects of breed and dietary regimen on reproductive traits for PP1, PP2 and PP3 are presented in tables 1, 2 and 3, respectively. Holsteins tended to have longer intervals to estrus (P>.1) and had longer intervals to conception (P.25) than Herefords during PP1, but breed did not affect these intervals in subsequent postpartum periods. Cows fed H diets had shorter intervals to estrus and conception in PP1 and PP2, but not in PP3)There were breed diet interactions for interval to estrus (P.6) in PP1 and PP2 (P.1) and for interval to conception (P.1) in PP1; the differences in mean intervals to estrus or conception between H and L groups were greater for Holsteins than for Herefords. Though not significant (P>.1), Herefords fed L diets tended to have shorter service intervals than the other groups in PP1 and PP2. Spearman's correlation coefficients between intervals to first estrus and service interval for PP1 and PP2 were -.46 (P.25) and -.21 (P>.25), respectively. Correlations between intervals to estrus and conception for PP1 and PP2 were.77 (P.1) and.7 (P.OOl). Body weight, TDN and FCM were signffi- Downloaded from jas.fass.org at University of Florida on June 1, 211

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8 1464 HANSEN ET AL. cantly greater for Holsteins than Herefords and for H than L cows in all postpartum periods (tables 1, 2 and 3). Holsteins lost weight from calving to 56 d postpartum during PP1 and PP3, while Herefords gained weight during the same periods. These differences were significant. During PP2, both groups gained weight. Diet did not significantly affect ABW during any postpartum period. There were breed diet interactions for FCM in PP1 (P.1), PP2 (P.5) and PP3 (P.5). Differences between H and L groups were greater for Holsteins than for Herefords. The Herefords receiving L diets were probably producing milk at a level near their maximum potential while Holsteins responded to the higher level of TDN in the diet by increasing milk production. Milk production is determined partially by amount of milk removed from the udder (Schmidt et al., 1964). Therefore, the capacity of a calf to consume milk could have affected the amount of milk produced. Holsteins fed H diets were the only group that did not increase in FCM production between PP2 and PP3 (tables 2 and 3), perhaps because the level of milk production in this group was determined by the capacity of their calves. Cows in other groups may have produced below their calves' ability to consume milk. Exp. 2. There were no significant effects of breed of sire on intervals to first ovulation, first estrus or conception in PP1 (table 4). Cows fed L diets had longer (P.lO) intervals to first estrus than H cows and longer intervals to first ovulation, though this effect was not significant. There were no significant breed-of-sire x diet interactions, but the influence of diet on intervals to first ovulation and estrus tended to be greater for HF and AF than for SF and CF. During PP2, breed of sire did not significantly influence intervals to ovulation, estrus or conception (table 5). Interval to ovulation was longer (P.5) for L than for H cows. There was a breedof-sire x diet interaction (P.1) as the interval to estrus was longer for L than H cows in the HF breed-of-sire group, but not in other breed-of-sire groups. There were no significant effects of breed of sire, diet or breed-of-sire x diet interactions in PP 3 for any traits. Service intervals were not significantly influenced by breed of sire or diet in any of the postpartum periods, though they tended to be greater for H- than L-fed cows in PPI and PP3. Spearman's correlation coefficients between service interval and interval to first estrus for PPI, PP2 and PP3 were-.49 (P.O1), -.35 (P.1) and -.16 (P.25), respectively. Correlations between intervals to first estrus and conception for PP1, PP2 and PP3 were.59 (P.O5),.23 (P.IO) and.45 (P.5), respectively. Breed of sire significantly influenced BW in all postpartum periods (tables 4, $ and 6); CF were heavier than AF and HF in PP1, PP2 and PP3 and heavier than SF in PP2, and SF were heavier than HF in PP1 and PP2. Cows fed the H diets were heavier at calving than L-fed cows for PP2 (P.1) and for PP3 (P.1), but not for PP1 (P>.1). Change in body weight was not significantly affected by breed of sire in any postpartum period, but tended to be greater in H-fed cows than L-fed cows in PP1 (P.8), PP2 (P.6) and PP3 (P>.1). The TDN intake was greater (P.1) for SF and CF cows than for HF and AF cows in PP1 and PP3 (tables 4 and 6). In PP2, SF cows consumed significantly more TDN than HF and AF cows, and CF cows consumed more TDN than HF cows. Cows fed the H diet consumed more TDN than L-fed cows in all postpartum periods. There were breed- of- sire x diet interactions on TDN intake in PP1 (P.25) and PP3 (P.8). In PP1, H-fed cows consumed more TDN than L-fed cows except in the SF breed-of-sire group, in which the opposite occurred. In PP3, H-fed cows consumed more TDN than L-fed cows in all breed-of-sire groups, but the differences between dietary regimens were greatest in CF cows and least in AF cows. Fat corrected milk was not influenced by breed of sire in PPl (table 4) or PP2 (table 5), but was greater (P.5) for SF than for HF cows in PP3 and greater in H- than L-fed cows for PPl (P.1) and PP3 (P.25). In PP2, there was a breed-of-sire x diet interaction (P.1); FCM was greater in H- than in L - fed cows in all breed - of- sire groups except CF, in which the opposite occurred. The results for PPl of partitioning the variance of interval to first estrus in a multiple regression analysis are presented in table 7. Because diet level influenced interval to first estrus, the data were analyed within diet group. Cows fed the H diet that calved at dates further from July 2 had longer intervals Downloaded from jas.fass.org at University of Florida on June 1, 211

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11 FACTORS AFFECTING POSTPARTUM REPRODUCTION IN COWS 1467 'N 'i"l 4-I +I +I 4"I +I 44 +I X r,.o :> "1 4"1 ~ m r~ 4"1 4"1 +I +I 4-1 4"I +I +I " f- 1:1 X +I +I +I +I +I 4,-I ~ 44 ~ u,,, +I 4-I +I 4-I +I 4"1 +I 4"I,-1 o [.., +I +I +I +I +I "14 +I +I " ~, +, ~ ~ ~ ~ Downloaded from jas.fass.org at University of Florida on June 1, 211

12 1468 HANSEN ET AL. TABLE 7. RELATIONSHIPS BETWEEN TRAITS AND INTERVAL TO ESTRUS AFTER FIRST CALVING: REGRESSION AND CORRELATION COEFFICIENTS (EXP. 2) a,b High diet Traits from to 56 d postpartum Body weight Change in Fat- corrected miik' at calving body weight TDN intake production Type of coefficient (BW) (ABW) (TDN) (FCM) CDI c R 2 Partial regression Standard partial regression Correlation ABW -.47** TDN.73****.3 FCM.59*** -.62****.24 CDI *.25.44*.25 Low diet Partial regression **.2* ** Standard partial regression " *.49" ** Correlation ABW f TDN.79**** FCM CDI ***.63***.62***.63*** aregression equation: Interval to first estrus -- intercept + b t BW + b 2 ABW + b 3 TDN + b 4 FCM + b s CDI + error. bthere was heterogeneity of regression between high and low dietary levels (P.5). CCalving date index, where = July 2 and 182 = January 1 and December 31. "['P.1. *P.5. **P.25. ***P(.O1. ****P.1. to first estrus (P.5). No other factors considered were significantly related to interval to first estrus in the H group and the coefficient of determination was small. Cows fed the L diet that consumed less TDN, produced more milk and calved at dates further from July 2, had longer intervals to first estrus. Body weight at calving could have influenced duration of the postpartum anestrus in the L group indirectly because TDN intake and BW were positively correlated (table 7). Total digestible nutrient intake was probably related to interval to estrus in L, but not H-fed cows because the higher energy density of the diets fed the latter group ensured feed intake adequate for optimal reproduction for most cows. Data from PP2 and PP3 were pooled across diet level because interval to first estrus was not significantly influenced by diet. None of the regression coefficients in either equation were significant and coefficients of determination were small (R 2 =.9 for PP2 and.16 for PP 3). The average lengths of gestation periods of the first, second and third calves (G1, G2, G3) are presented in table 8. The first gestation and G2 tended to be the longest for CF cows, followed by HF, SF and AF, but the differences were not significant. There was a significant influence of breed of sire on G3; CF and HF cows had longer gestation periods than AF cows. The effect of diet on G1 was not significant, but G1 tended to be shorter for H than for L cows. There was a breed - of- sire x diet interaction (P.5) for G1 because HF and SF crossbreds fed the Downloaded from jas.fass.org at University of Florida on June 1, 211

13 FACTORS AFFECTING POSTPARTUM REPRODUCTION IN COWS 1469 TABLE 8. INFLUENCE OF BREED OF SIRE AND DIET ON GESTATION LENGTH (EXP. 2) a..... ', ',.',T' Gestation period (d) Breed of sire Dietary level First Second Third Hereford High d 286. ~ , Angus High Simmental High ,9 Chianina High Hereford Low Angus Low Simmental Low 28.$ :!: Chianina Low Breed Hereford b b 289,5 c Angus b b b Simmental b b bc Chianina 28.9 b b c Diet High b b b Low 279,1b b 289. b aleast-squares means + SE. b,cmean s with different superscripts differ at (P.5). All other ~lifferences were nonsignificant. dthere was a breed diet interaction (P.5). low diets had longer gestation periods than cows fed the high diets. Cows fed the L diet had shorter gestation periods for CF crossbreds, and AF crossbreds were not affected by dietary regimen. Diet did not significantly affect length of G2 or G3. Discussion In Exp. 1, the differences between H- and L-fed cows in intervals from calving to first estrus were greater for Holsteins than for Herefords. Similar breed x nutrition interactions have been observed comparing Holsteins to Herefords and Hereford x Holsteins (Kropp et al., 1973) and high milkproducing to low milk-producing Holsteins (Whitmore et al., 1974). Breed differences between Holsteins and Herefords other than milk production could also have influenced the interaction of breed with diet. No interactions were observed in Exp. 2 when comparing breeding groups chosen for high and low milk production, but in PP1, cows with greater milk production had longer intervals to first estrus for L- fed, but not for H-fed cows, indicating a diet x milk production interaction. An influence of level of milk production on interval to first estrus has been reported for dairy cows (Menge et al., 1962; Saiduddin et al., 1968). Our results suggest that level of milk production also influenced duration of postpartum anestrus in cows suckling calves when they were fed a diet of chopped hay. It was not possible to determine whether increased milk production affects return to estrus by increasing nutrient demands, by increasing frequency of suckling (Randel and Welker, 1977) or by some other mechanism. These interactions in reproductive function indicate that the adaptability of breeds varies with environmental conditions. When nutrients are limited, cows with genotypes for high milk production will have longer postpartum anestrous periods relative to cows with genotypes for low milk production. Ample nutrient availability will shorten interval to first estrus for high milk producing COWS. In Exp. 2, breed x diet interactions were not significant in PP1 for intervals to ovulation and estrus, but the influence of diet tended to be greater in the small (HF and AF) breedof-sire groups than in the large (SF and CF) breed-of-sire groups. In PP2, interval to first estrus was influenced by dietary regimen only for HF cows. The ability of the large breeds to consume more TDN may have ac- Downloaded from jas.fass.org at University of Florida on June 1, 211

14 147 HANSEN ET AL. counted for the interaction because TDN intake seemed to influence reproductive function only on the low diet. Our results differed from others on the influence of nutrition (Wiltbank et al., 1962, 1964; Dunn et al., 1969; Kropp et al., 1973; Whitmore et al., 1974; Bellows and Short, 1978; Holness et al., 1978) in that diet did not affect reproduction as the animals became older. This was probably in part due to different methods employed to alter levels of nutrition. We fed chopped hay-grain diets essentially ad libitum and controlled TDN content of the diets, while in the studies mentioned above feed intake was controlled. As the number of parities and age increased, TDN intake also increased as the animals continued to grow in sie. Hohenboken et al. (1972) found that cows fed in the same manner as in our experiments, required the same percentage of TDN for maintenance from calving to 24 d postpartum, regardless of parity. This would indicate that as TDN consumption increased with parity, more TDN would be available for uses other than maintenance, including reproduction. As a consequence, the L diet may have provided sufficient TDN for the cows to function reproductively similar to H-fed cows in PP2 and PP3. Greater availability of TDN for uses other than mainte - nance as parity and age increased was indicated by greater weight gains after calving for L-fed cows in Exp. 2 during PP2 and PP3 as compared with PP1. The greatest difference in TDN composition of the diets existed prior to 449 (Exp. 1) or 42 d (Exp. 2). Because heifers in Exp. 2 were bred at puberty, those reaching puberty at younger ages received rations higher in TDN during a greater part of their fir.ct gesta - tion than heifers reaching puberty at older ages. Thus, differences between TDN content of the rations between H and L groups may have been greater during the gestation preceding first calving than differences between diets in the gestations preceding subsequent calvings, because H heifers were younger at first breeding. Diet may also have had a greater effect after first calving than after subsequent calvings if animals bred at younger ages calved in a different season than animals bred at older ages because season of calving affected reproduction during PP1 (P. J. Hansen and E. R. Hauser, unpublished data). In Exp. 1, the last diet changes were made before first breeding and most animals gave birth to their first calf in one season. However, it is possible that diet before first breeding may have affected reproduction after first and second calving, but not later calving, through mechanisms other than the ones mentioned here. Neither body weight at calving nor changes in body weight postpartum were significantly related to duration of the postpartum anestrus in Exp. 2. Failure to find relationships between these traits and interval to estrus differs from findings of others (Menge et al., 1962; Whitman et al., 1975; Holness et al., 1978; Stevenson and Britt, 198). In our experiments, service interval was not affected by diet, in contrast to findings (Wiltbank et al., 1964; Folman et al., 1973) that cows on a high plane of nutrition had higher conception rates than cows on a low plane of nutrition. The tendency in our experiments was for the service interval to be shorter in L-fed than H-fed cows when dietary regimen influenced interval to estrus, probably because H-fed cows were bred sooner postpartum. Our data and those of others (Van Demark and Salisbury, 195; Shannon et al., 1952; Whitmore et al., 1974) demonstrated that breeding early in the postpartum period was associated with lower fertility, but the high correlations between intervals to first estrus and conception suggest that cows that exhibited estrus earlier conceived earlier, even though their fertility was lower. Intervals from calving to first estrus were longer for cows that had calving dates further from midsummer. A function of calving date was included in the multiple regression analysis to determine the influence of other traits independent of season of calving. Gestation appeared to be longer for CF cows than for other crossbred groups in Exp. 2. A survey by Preston and Willis (197) indicated long gestation periods for Chianinas. Cows fed L diets had longer average G1 for HF and SF cows. Kress et al. (1971a) noted cows fed low levels of nutrition had longer gestations than cows fed high levels of nutrition. The first gestation (G1) was shorter than G2 and G3, possibly because of age or parity effects or because the breeds of sire of the calves being gestated differed. Positive correlations of gestation length with age of dam have been reported by Jafar et al. (195) and Burfening et al. (1978). Downloaded from jas.fass.org at University of Florida on June 1, 211

15 FACTORS AFFECTING POSTPARTUM REPRODUCTION IN COWS 1471 In conclusion, diet interacted with breed and age (or parity) to influence postpartum reproduction; the difference between H and L groups was greater for Holsteins than for Herefords, tended to be greater for HF and AF than SF and CF cows and was greater following first calving than after subsequent calvings. Level of milk production influenced postpartum reproduction when nutrition was limited. The large populations of dairy cows offers the opportunity to produce beef x dairy crossbreds that can be used as beef cows because dairymen, at times, breed first-calf heifers or low-producing cows to beef bulls. The breeding of a portion of the dairy cattle population in this manner would allow beef producers to take advantage of an earlymaturing, high milk-producing, crossbred female in a terminal sire system of crossbreeding, without the necessity of maintaining large numbers of srtaighthred beef cows. The interaction of diet with breed in Exp. 1 and milk production in Exp. 2 indicated that nutrition must be adequate if dairy breeds and their crosses are to be used as beef cows, especially if heifers are bred at their pubertal estrus. Because diet was not an important determinant of postpartum reproductive performance after PP1 in Exp. 2, the inference can be drawn that nutrition is not as critical a factor in reproductive efficiency for beef x Holstein crosses as it is for straightbred Holsteins. When nutrition of postpartum cows is limited by energy content of ration rather than amount of feed available, postpartum reproductive performance may not be affected in cows capable of consuming large amounts of feed. Nutritional regiflaen may then be a more important determinant of postpartum reproduction in primiparous females than multiparous females. Literature Cited Bellows, R. A. and R. E. Short Effects of precalving feed level on birth weight, calving difficulty and subsequent fertility. J. Anita. Sci. 46:1522. Burfenlng, P. J., D. D. Kress, R. L. Friedrich and D. D. Vaniman Phenotypic and genetic relationships between calving ease, gestation length, birth weight and preweaning growth. J. Anita. Sci. 47:595. de Kruif, A An investigation of the parameters which determine the fertility of a cattle populaion and of some factors which influence these parameters. Tijdschr. Diergeneesk. 1:189. Dunn, T. G., J. E. Ingalls, D. R. immvrman and J. N. Wiltbank Reproductive performance of 2-year-old Hereford and Angus heifers as influenced by pre- and post-calving energy intake. J. Anim. Sci. 29:719. Folman, Y., M. Rosenberg,. Her and M. Davidson The relationship between plasma progesterone concentration and conception in post-partum dairy cows maintained on two levels of nutrition. J. Reprod. Fertil. 34:267. Foote, W. D., E. R. Hanser and L. E. 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