| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
ANIMAL PRODUCTION |
University of Nebraska, West Central Research and Extension Center, North Platte 69101
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
|---|
Key Words: Beef Heifers • Breeding Dates • Heifer Development • Reproduction • Target Weights
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
|---|
) indicated that puberty can be expected to occur at a genetically predetermined size among individual animals, and only when heifers reach their predetermined 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.
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 |
|---|
|
TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
|---|
Gelbvieh, Simmental, Angus, 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.
|
) 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; 543.40 kg ± 65.32 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-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 x treatment interaction. Pregnancy rates were analyzed using 
2 procedures (Steel and Torrie, 1980).
|
Results and Discussion |
|---|
|
TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
|---|
; 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 high-gain 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 x 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.
|
. 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 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.
|
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 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 x 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 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, probably because of diminishing nutrient values in native forages during the breeding season.
|
Implications |
|---|
|
TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
|---|
|
Footnotes |
|---|
2 Correspondence: 461 W. University Dr. (phone: 308-532-3611; fax: 308-532-3823; e-mail: rfunston2{at}unl.edu).
Received for publication March 11, 2004. Accepted for publication June 18, 2004.
|
Literature Cited |
|---|
|
TopAbstractIntroductionMaterials and MethodsResults and DiscussionImplicationsLiterature Cited |
|---|
This article has been cited by other articles:
|
|
 |
|
  J. L. Martin, K. W. Creighton, J. A. Musgrave, T. J. Klopfenstein, R. T. Clark, D. C. Adams, and R. N. Funston Effect of prebreeding body weight or progestin exposure before breeding on beef heifer performance through the second breeding season J Anim Sci, February 1, 2008; 86(2): 451 - 459. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. J. Roberts, S. I. Paisley, T. W. Geary, E. E. Grings, R. C. Waterman, and M. D. MacNeil Effects of restricted feeding of beef heifers during the postweaning period on growth, efficiency, and ultrasound carcass characteristics J Anim Sci, October 1, 2007; 85(10): 2740 - 2745. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. E. Grings, T. W. Geary, R. E. Short, and M. D. MacNeil Beef heifer development within three calving systems J Anim Sci, August 1, 2007; 85(8): 2048 - 2058. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
