Comparison of target breeding weight and breeding date for replacement beef heifers and effects on subsequent reproduction and calf performance 1

Transcription

1 Comparison of target breeding weight and breeding date for replacement beef heifers and effects on subsequent reproduction and calf performance 1 R. N. Funston 2 and G. H. Deutscher University of Nebraska, West Central Research and Extension Center, North Platte ABSTRACT: A 3-yr study was conducted with spring-born heifers (n = 240) to determine the effects of developing heifers to either 55 or 60% of mature BW at breeding on reproduction and calf production responses. A concurrent study was also conducted with summer-born heifers (n = 146) to examine effects of breeding heifers with the mature cow herd or 1 mo earlier on reproduction and calf production variables. Spring-born crossbred heifer calves were weaned and developed on two different levels of nutrition to achieve the desired prebreeding BW. Summer-born heifers were developed to similar target breeding BW (60% of mature BW) to begin calving either 1 mo before (May) or at the same time as the mature cowherd (June). Blood samples were taken before breeding to determine differences in estrous cyclicity. Pregnancy rates through the fourth pregnancy were determined. Cow and calf production variables were evaluated through the third gestation. Spring-born heifers reached 53 or 58% of mature BW at breeding and had similar reproduction and first calf production traits between the two groups. Calving difficulty with the second calf was greater (P < 0.05) for heifers developed to 58% of mature BW at breeding. Subsequent second calf weaning weight and ADG were decreased (P < 0.05) for heifers developed to 58% of mature BW at breeding. Feed costs were $22/heifer less for heifers developed to 53% of mature BW. Summer-born first-calf heifers calving in June had less (P < 0.01) calving difficulty than did heifers calving in May; however, calf birth weights were similar. Breeding summer-born heifers 1 mo before the cowherd did not influence pregnancy rates over three calf crops; however, first calf adjusted weaning weights and ADG were greater for calves born earlier. Development costs were $11/heifer more for heifers developed to calve in May vs. June. Developing spring-born heifers to 53% of mature BW did not adversely affect reproduction or calf production traits compared with developing heifers to 58% of mature BW, and it decreased development costs. Breeding summer-born heifers before the cowherd increased heifer development costs, increased calving difficulty, and improved calf performance, but had no effect on pregnancy rates. Key Words: Beef Heifers, Breeding Dates, Heifer Development, Reproduction, Target Weights 2004 American Society of Animal Science. All rights reserved. J. Anim. Sci : Introduction Proper development of replacement heifers is critical and needs to be accomplished at low costs without sacrificing performance. Heifers should be managed to reach puberty early, conceive early in the first breeding season, calve unassisted, and breed back early for their second calf. Previous studies (Patterson et al., 1992) indicated that puberty can be expected to occur at a genetically predetermined size among individual animals, and only when heifers reach their predetermined 1 A contribution of the Univ. of Nebraska Agric. Res. Div., Lincoln 68583, Journal Series No Correspondence: 461 W. University Dr. (phone: ; fax: ; rfunston2@unl.edu). Received March 11, Accepted June 18, target BW can high pregnancy rates be obtained. Recommended guidelines generally have been 60 to 66% of mature BW in beef heifers, depending on frame size (Patterson et al., 1992). In contrast, some studies do not support a critical BW or composition hypothesis in heifer development (Brooks et al., 1985). Inconclusive results not only exist regarding the appropriate target BW, but also the appropriate time heifers should be placed with bulls in relation to the mature cowherd. Common practice is to breed heifers before the cowherd so they have a longer rebreeding period. This development program requires additional resources if heifers are developed to the same target BW because an accelerated rate of gain is needed to reach the target BW earlier. As summer calving has gained interest in the Midwest, development programs that allow heifers to conceive early as yearlings and rebreed for a second calf at the lowest cost possible are needed. 3094

2 Heifer breeding weights and dates 3095 Table 1. Winter diets (as-fed basis) and costs for developing spring- and summer-born heifers over three years Spring a Summer a Low High August September Item gain gain breeding breeding Diets Meadow hay, kg Pellets, kg b Corn, kg Feeding period, d Costs c Feed costs, $/d Feed costs, total feeding period, $ a Feeding periods were mid-december to mid-may for spring-born heifers and mid-january to mid-may for summer-born heifers. b Pellet composition: 48% wheat middlings; 40% soybean hulls; 5% cottonseed meal; 5% cane molasses plus vitamin-mineral mix, urea; and 88 mg/kg Rumensin. c Average prices were as follows: hay, $0.044/kg; supplements, $0.149/kg; cracked/delivered corn, $0.108/ kg. The objectives of this study were 1) to compare development of spring-born heifers at two prebreeding target BW (55 or 60% of mature BW) and to determine effects on reproduction and cow and calf productivity, and 2) to develop summer-born heifers to similar target BW, but different breeding dates (1 mo before or same date as mature cowherd) and determine effects on reproduction and subsequent cow and calf productivity. Materials and Methods A 3-yr study was initiated using heifer calves selected from the MARC II (¹ ₄ Gelbvieh, ¹ ₄ Simmental, ¹ ₄ Angus, ¹ ₄ Hereford) cowherds at the Gudmundsen Sandhills Laboratory near Whitman, NE. In 1997, 1998, and 1999, approximately 80 spring-born (calved in March and April) heifers and 50 summer-born (calved in June and July) heifers were selected each year as replacements for the spring and summer cowherds. The genetic composition was similar in each herd and the same bulls were used in both herds each year. Each year, weaned heifers were placed in drylot pens by treatment groups for the winter feeding phase. Diets and feed costs for the winter feeding period are presented in Table 1. Heifers were fed meadow hay, wheat middlings, soybean hull-based pellets, and cracked corn in balanced diets to achieve the desired gains and target BW. Hay (9 to 10% CP; as-fed basis) was fed ad libitum in bale feeders. Pellets (20% CP) with Rumensin (88 mg/kg) and a vitamin-mineral mix were fed in bunks with cracked corn as needed. Heifers were weighed monthly and diets adjusted accordingly to obtain desired gains. After the winter-feeding phase, heifers were weighed, their BCS determined (1 to 9 scale, 1 = emaciated, 5 = moderate, 9 = obese) on May 15, and they were moved to native range for summer grazing. Before the breeding season, pelvic area was determined and blood samples were taken 10 d apart for progesterone analysis (Anthony et al., 1981) to determine cycling status. The pregnant heifers grazed subirrigated meadow regrowth during the fall, and were fed meadow hay and supplement (0.68 kg/d, 40% CP; as-fed basis) during the winter. Two-year-old cows were managed separately from the mature cowherd and grazed native range during the summer. All pregnant 3-yr-old cows were placed with the mature cowherds and fed and managed with them thereafter. All nonpregnant heifers and cows and cows that lost calves were removed from the study each year. Experiments were conducted under approved University of Nebraska Animal Care and Use Committee guidelines. Spring-Born Heifers Weaned spring heifers were allotted to treatment by age and BW in mid-december. They were assigned either to a low- or high-gain treatment. The low-gain treatment was fed to reach a prebreeding target BW of 300 kg (55% of mature BW) by May 15. The high-gain treatment was fed to reach a prebreeding target BW of 327 kg (60% of mature BW) by May 15. Target weights were calculated from the previous year average fall cow BW (4 yr and older; kg ± SD; n = 266). Four Angus bulls were placed with the spring-born heifers on May 20 for a 45-d breeding season. Approximately 60 d after the end of the breeding season, heifers were palpated for pregnancy, and BW and BCS were recorded. Calving began approximately March 1. Calving difficulty and calf birth weight were recorded and calving assistance provided as needed. Calving difficulty was evaluated on a 1 to 5 scale, where 1 = no assistance, 2 = easy pull, 3 = mechanical pull, 4 = hard mechanical pull, and 5 = Caesarean section. After calving, spring-

3 3096 Funston and Deutscher born heifers were fed meadow hay plus supplement (0.68 kg, 40% CP; as-fed basis). The 2-yr-old cows were exposed to MARC II bulls on June 5 each year for rebreeding. Calves from 2-yr-old cows were weaned in early September. Calves from the 3- and 4-yr-old cows were weaned in mid-september at approximately 175 d of age. Summer-Born Heifers Weaned heifers were allotted to treatment by age and BW in mid-january. They were assigned either to an August or September breeding group. These heifers were developed to reach target BW of approximately 327 kg or 60% of mature BW by the beginning of breeding season. Summer-born heifers were exposed to bulls beginning either August 5 (1 mo before the mature cowherd) or September 5 (same date as the mature cowherd). Four Angus bulls (the same bulls used with the spring-born heifers) were used on the summer-born heifers for 45 d. Approximately 60 d after the end of each breeding season, heifers were palpated for pregnancy, and BW and BCS were recorded. Heifers were fed meadow hay plus supplement (0.68 kg, 40% CP; as-fed basis) during the winter until May 15, and then moved to summer range, where they calved. Calving began approximately May 15 for heifers exposed to bulls in August, and on June 15 for heifers exposed to bulls in September. Calving difficulty (as described for spring-born heifers) and calf birth weight were recorded on all heifers, and calving assistance provided as needed. The summer-born 2-yr-old cows were placed with MARC II bulls on September 5. The 2-yr-old cows were fed 0.45 kg/d of 48% CP cubes (as-fed basis) during the breeding season in 1999; and in 2000 and 2001, the 2- yr-old cows were fed cubes (0.45kg/d; 48% CP; as-fed basis) 45 d before and during the breeding season. Calves from summer-born 2-yr-old cows were weaned in late November. Calves from the 3- and 4-yr-old cows were weaned in early January at approximately 200 d of age. Non-categorical data were analyzed using GLM procedures of SAS (SAS Inst., Inc., Cary, NC). Effects included in the model were treatment, year, and the year treatment interaction. Pregnancy rates were analyzed using χ 2 procedures (Steel and Torrie, 1980). Spring-Born Heifers Results and Discussion Heifers on the high-gain diet gained approximately 0.14 kg/d more than on the low-gain diet (0.64 vs kg/d; Table 2; P < 0.01) and cost $22/heifer more during the 155-d wintering period (Table 1). At prebreeding, the high-gain heifers weighed 24 kg more (P < 0.01) than the low-gain group and had 0.4 unit greater condition score (6.0 vs. 5.6 BCS; Table 2; P < 0.01). The highgain heifers were 58% of mature BW, and the low-gain heifers were 53% of mature BW at prebreeding. The percentage of heifers cycling before breeding was 11% greater for the high-gain heifers (85 vs. 74%; P < 0.01). The 45-d pregnancy rate was not different between groups (92 vs. 88% for low- and high-gain, respectively). Treatment year interactions were not significant for any of the spring-born heifer data, except for ADG during the development period. This was due to a change in magnitude of difference in gain during the third year. The heifers that were developed to 58% of mature weight gained approximately 0.2 kg/d more than the heifers developed to 53% of mature weight during the first 2 yr; however, the difference in gain the third year was only 0.1 kg/d. Data for calving and weaning of the first calf, and rebreeding of the spring-born cows are presented in Table 3. The high-gain cows were heavier (P < 0.01) at calving but not different at weaning. Average calf birth date, calf birth weight, and calving difficulty were similar for both groups. Calf gain and adjusted 205-d weights were also similar for both groups of cows, indicating that milk production was probably similar. Percentage of cows rebreeding for their second calf did not differ between groups (Table 3). Calving and weaning data for the second and third calf and cow BW, BCS, and rebreeding performance are presented in Table 4. Calving difficulty score was greater (P < 0.05) for the high-gain cows calving for the second time. Subsequent calf performance (ADG and adjusted 205-d weight) of calves from the high-gain cows was also decreased (P < 0.05). The reason for the difference in calving difficulty is not understood because birth weights of calves and pelvic area before first breeding were similar between groups. The difference in subsequent calf performance may be related to increased dystocia. Bellows et al. (1988) reported that calves from cows given early calving assistance had greater ADG and weaning weights than calves from cows given late calving assistance. Alleviation of stress either by decreased calving difficulty or early assistance may have similar physiological effects on the cow and calf. Developing heifers to 53% of mature BW did not adversely affect reproduction or calf production traits compared with developing heifers to 58% of mature BW. However, costs were increased when heifers were developed to the greater breeding BW. Summer-Born Heifers Heifers averaged 183 kg at the beginning of the experimental feeding period (mid-january, Table 2). Heifers developed to be placed with bulls in August gained 0.14 kg/d more than heifers developed to be placed with bulls in September; feed cost was $11/heifer greater for the August vs. the September group ($66 vs. $55; Table 1). Heifer development and first pregnancy results are presented in Table 2. The August heifers (263 kg) were

4 Heifer breeding weights and dates 3097 Table 2. Heifer development and breeding results for spring- and summer-born heifers Spring Summer Low High August September Trait gain gain SEM breeding breeding SEM No. of heifers Beginning BW, kg a Winter ADG, kg/d 0.50 c 0.64 d c 0.54 d 0.01 Prebreeding wt, kg 289 c 313 d c 330 d 3.01 Prebreeding BCS b 5.6 c 6.0 d Pelvic area, cm c 181 d 1.60 Cycling before breeding, % 74 c 85 d Start of breeding season, Julian d Pregnant in 45 d, % a Heifer development began in mid-december for spring-born heifers and mid-january for summer-born heifers. b Scoring system 1 to 9: 1 = emaciated; 5 = moderate; 9 = obese. c,d Treatment means in row within season that do not have common superscripts differ, P < heavier (P < 0.01) by mid-may than the September heifers (249 kg); however, the September heifers were heavier at breeding (319 and 330 kg for August and September, respectively; P < 0.01). Both groups BW were approximately 60% of mature BW. The percentage of heifers cycling before breeding was similar for both groups. Treatment year interactions were not significant for any of the summer-born heifer data. The 45-d yearling pregnancy rate did not differ (P > 0.10) for September (93%) and August heifers (88%). Calf birth weights were similar for the 2-yr-old cows (Table 3). However, calving difficulty score was greater (P < 0.01) for the cows calving in May (1.20) than those calving in June (1.0). The prebreeding pelvic area (Table 2) was larger (P < 0.05) for the June-calving cows, which may have had some influence on calving difficulty. When comparing calving difficulty between the spring- and summer-born heifers (March vs. May vs. June), cows calving in June seemed to have less calving difficulty than those calving in March or May. This difference was not due to smaller calf birth weights; June-calving heifers would also be approximately 1 mo. older when calving, which may have decreased incidence of dystocia. Other factors influencing calving difficulty may have included warmer temperatures, less heifer stress, more pelvic relaxation, better nutrition on green grass, and more heifer exercise (Colburn et al., 1997). Calf gain to weaning was greater (P < 0.01) for the calves of the 2-yr-old May-calving cows. Actual calf weaning weights were 32 kg heavier (P < 0.01), and adjusted 205-d weights were 10 kg heavier (P < 0.01) for the May-calving cows (Table 3). A review by Sprott et al. (2001) reported similar results in Montana and Table 3. First year calf production and rebreeding of spring- and summer-born heifers Spring Summer Low High August September Trait gain gain SEM breeding breeding SEM No. of heifers Calving season began, Julian d Precalving BW, kg 415 d 426 e Precalving BCS a Calf birth date, Julian d d 174 e 1.36 Calf birth wt, kg b Calving difficulty score c d 1.0 e 0.04 Weaning date, Julian d Actual calf weaning wt, kg b d 148 e 2.5 Calf ADG, kg/d b d 0.74 e d adjusted calf weaning wt, kg b d 186 e 2.61 Cow BW at weaning, kg Cow BCS at weaning a 5.1 d 5.3 e Cows pregnant with second calf, % a Scoring system 1 to 9: 1 = emaciated; 5 = moderate; 9 = obese. b Calf BW were adjusted for sex. c Scoring system 1 to 5: 1=noassistance; 2 = easy pull; 3 = mechanical pull; 4 = hard mechanical pull; and 5 = Caesarean section. d,e Treatment means in row within season that do not have common superscripts differ, P < 0.01.

5 3098 Funston and Deutscher Table 4. Second and third year calf production and rebreeding rates of spring- and summer-born heifers Spring Summer Low High August Sept. Trait gain gain SEM breeding breeding SEM No. of cows Calf birth date, second calf, Julian d Calving interval, d d 369 e 2.20 Calf birth wt, kg a Calving difficulty score b 1.07 d 1.17 e Actual calf weaning wt, kg a Calf ADG, kg/d a 0.92 d 0.88 e d adjusted calf weaning wt, kg a 224 d 215 e Cow BW at weaning, kg 448 d 461 e Cow BCS at weaning c Cows pregnant with 3rd calf, % No. of cows Calf birth date, third calf, Julian d Calving interval, d Calf birth wt, kg a Calving difficulty score b Actual calf weaning wt, kg a Calf ADG, kg/d a d adjusted calf weaning wt, kg a Cow BW at weaning, kg d 442 e 7.03 Cow BCS at weaning c Cows pregnant with fourth calf, % a Calf BW were adjusted for sex. Calves from spring herd were weaned at approximately 175 d, and summer-born calves were weaned at approximately 200 d. b Scoring system 1 to 5: 1=noassistance; 2 = easy pull; 3 = mechanical pull; 4 = hard mechanical pull; and 5 = Caesarean section. c Scoring system 1 to 9: 1 = emaciated; 5 = moderate; 9 = obese. d,e Treatment means in row within season that do not have common superscripts differ, P < Nebraska, where calves born in June were lighter at weaning at a constant age than calves born earlier; however, conflicting results were reported in studies in other regions of the United States. The effect of calving date on calf production is likely a function of nutrient availability before weaning. Heifers calving in May were allowed a longer postpartum recovery period and subsequently had a greater (P < 0.01; Table 4) calving interval than June-calving heifers. However, pregnancy rates for the second calf were not different (78 and 82%, for May and June calving, respectively). Rebreeding rates were numerically decreased compared with the spring-calving heifers, even though 2-yr-old cows were supplemented (0.45 kg/ d; 48% CP; as-fed basis) during the breeding season the first year and an additional 45 d before breeding the next 2 yr. This was probably due to the low nutritional value of mature grass during the September and October breeding season (Adams et al., 1996). Subsequent calf performance and pregnancy rates did not differ through the next two calf crops (Table 4); however, cow BW was heavier (P < 0.05) for heifers developed to breed and calve earlier (May vs. June). This is similar to the spring-born heifer development study, in which heifers developed on an accelerated rate of gain had heavier mature BW. There may be an opportunity to manipulate mature BW and nutrient requirements by heifer development regimen. Previous studies support this hypothesis, where limit feeding improved feed efficiency (Galyean, 1999). Breeding heifers to calve 1 mo before the cow herd did not affect rebreeding rates; however, May-calving 2-yr-old heifers had heavier actual and adjusted 205- d calf weaning weights compared with June-calving heifers. June-born calves had birth weights similar to those of May-born calves, but less calving difficulty was experienced with June calving. Developing heifers to calve with the cowherd was accomplished on a decreased rate of gain and decreased costs compared with developing heifers to calve 1 mo earlier. The economic impact of this system of heifer development was not evaluated completely and will likely depend on marketing strategies for the calf. Many producers that have switched to a later calving system have simultaneously integrated a yearling retention program to minimize the adverse effects of decreased weaning weights (May et al., 1999). Future studies are needed to evaluate the effects of decreased weaning weights on subsequent backgrounding and yearling strategies. Pregnancy rates of summer-born heifers were satisfactory at yearling breeding, but unsatisfactory at rebreeding. Additional supplementation is required to improve rebreeding rates in summer-calving cows, proba-

6 Heifer breeding weights and dates 3099 bly because of diminishing nutrient values in native forages during the breeding season. Implications Opportunities may exist to lower heifer development costs by lowering traditional target breeding weights or by breeding heifers the same time as the cowherd (compared with breeding and calving before the cowherd). Both of these systems allow for development on a lower rate of gain and may decrease feed costs; however, the interaction of these two practices remains to be evaluated. Mature size of a heifer developed at a lower postweaning rate of gain may also be decreased and may potentially affect nutrient requirements throughout the animal s lifetime. Literature Cited Adams, D. C., R. T. Clark, T. J. Klopfenstein, and J. D. Volesky Matching the cow with forage resources. Rangelands 18: Anthony, R. V., R. J. Kittok, E. F. Ellington, and M. K. Nielsen Effects of zeronal on growth and ease of calf delivery in beef heifers. J. Anim. Sci. 53: Bellows, R. A., R. E. Short, R. B. Staigmiller, and W. L. Milmine Effects of induced parturition and early obstetrical assistance in beef cattle. J. Anim. Sci. 66: Brooks, A. L., R. E. Morrow, and R. S. Youngquist Body composition of beef heifers at puberty. Theriogenology 24: Colburn, D. J., G. H. Deutscher, M. K. Nielsen, and D. C. Adams Effects of sire, dam traits, calf traits, and environment on dystocia and subsequent reproduction of two-year-old heifers. J. Anim. Sci. 75: Galyean, M. L Review: Restricted and programmed feeding of beef cattle Definitions, application, and research results. Prof. Anim. Sci. 15:1 6. May, G. J., L. W. Van Tassell, J. W. Waggoner, and M. A. Smith Relative costs and feeding strategies associated with winter/spring calving. J. Range Manage. 52: Patterson, D. J., R. C. Perry, G. H. Kiracofe, R. A. Bellows, R. B. Staigmiller, and L. R. Corah Management considerations in heifer development and puberty. J. Anim. Sci. 70: Sprott, L. R., G. E. Selk, and D. C. Adams Review: Factors affecting decisions on when to calve beef females. Prof. Anim. Sci. 17: Steel, R. G. D., and J. H. Torrie Principles and Procedures of Statistics: A Biometrical Approach. 2nd ed. McGraw-Hill Publishing, New York.