Beef Research News
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Reproductive tract scoring in beef heifers.
In this study, 272 beef heifers were studied from just before their first breeding season (15 October 2003), through their second breeding season and until just after they had weaned their first calves in March, 2005. This study was performed concurrently with another study testing the economic effects of an estrus synchronization protocol using prostaglandin. Reproductive tract scoring (RTS) by rectal palpation was performed on the group of heifers 1 d before the onset of their first breeding season. The effect of RTS on several fertility and production outcomes was tested, and the association of RTS with the outcomes was compared to that of other input variables such as BW, age, body condition score, and Kleiber ratio using multiple or univariable linear, logistic, or Cox regression. Area under the curve for receiver operating characteristic analysis was used to compare the ability of different input variables to predict pregnancy outcome. After adjustment for weight and age, RTS was positively associated with pregnancy rate to the 50-d artificial insemination season (P < 0.01), calf weaning weight (r = 0.22, P < 0.01), and pregnancy rate to the subsequent breeding season (P < 0.01), and negatively associated with days to calving (r = 0.28, P < 0.01). Reproductive tract scoring was a better predictor of fertility than was Kleiber ratio, and similar in its prediction of calf weaning weight. It was concluded from this study that RTS is a predictor of heifer fertility, compares well with other traits used as a predictor of production outcomes, and is likely to be a good predictor of lifetime production of the cow.
D. E. Holm, P. N. Thompson and P. C. Irons. The value of reproductive tract scoring as a predictor of fertility and production outcomes in beef heifers. Published online first on March 13, 2009. J. Anim Sci. 1910. doi:10.2527/jas.2008-1579
Weaning regimen effect on beef cow performance
The effect of shifting calf-weaning age on profiles of energy status (BW, BCS, rib and rump fat) and reproductive performance of beef cows was evaluated in a 3-yr study. Pregnant and lactating crossbred beef cows (n = 408), mainly of Angus and Hereford breeding, were stratified by age, and by sex and weight of their calves and assigned randomly into two treatments: weaning at approximately 180 d (early weaning) and normal weaning 45 d later (control). Cows were managed together on native range pastures and supplemented with harvested forage during the winter months. Cow BW, BCS, ribfat and rumpfat were measured periodically from early weaning through the next breeding. Reproductive performance was evaluated by calving intervals (CI), days from initiation of breeding to calving (BCI), retention in the herd, and adjusted 205-d weaning BW of the subsequent calf. Early-weaned cows had greater (P < 0.001) BW at normal weaning than control cows, but the overall pattern of cow BW did not differ (P > 0.05) among treatments. Peak and nadir BCS occurred at pre-calving and post-calving periods, respectively and were greater (P < 0.001) at each period in early-weaned than in control cows and in cows 5-yr-old than in younger cows. Patterns for rib fat and rump fat were nearly identical to those of BCS except for the 3-way interaction (P < 0.001) of treatment, age, and period on rumpfat. Mean CI (372.4 ± 2.1 d) and BCI (299.7 ± 1.9 d) were not affected (P = 0.42) by treatment, but varied (P < 0.001) with age of the cow. Age of cow accounted for 16% of total variation in CI and 12% of total variation in gestation length (P < 0.001). The intervals were longer (P < 0.001) in primiparous cows than in older cows. Early weaning decreased risk of culling in cows and thereby increased (P < 0.05) overall persistence by 11% over control cows. Earlier weaning of cows in the previous year increased (P < 0.001) weaning weight of the subsequent calf by 8.6 kg per cow per yr. Shifting weaning time increased storage of consumed energy as evidenced by increased rump fat, for use later during high-energy demand, ultimately improving overall productivity of the cow-calf system.
J. F. Odhiambo, J. D. Rhinehart, R. Helmondollar, J. Y. Pritchard, P. I. Osborne, E. E. Felton and R. A. Dailey. Effect of weaning regimen on energy profiles and reproductive performance of beef cows. Published online first on April 9, 2009 J. Anim Sci. 1910. doi:10.2527/jas.2008-1138
Body condition score affect on post-pubertal beef heifers
Twenty Simmental x Angus, half-sibling, post-pubertal heifers (initial BW 443 ± 9 kg) were allotted randomly into 2 treatment groups to evaluate if initial BCS affects response of the hypothalamic-pituitary-ovarian axis to metabolic signals elicited by energy restriction and repletion. During a preliminary feeding period, diets were formulated so that each heifer in the designated treatment would reach either a BCS of 5 (moderate condition; MOD) or a BCS of 7 (heavy condition; FAT). Once each heifer had reached desired BCS, diets were formulated to supply 30% of NEm requirements until each heifer became anestrous (serum concentrations of P4 < 1 ng/mL; restriction period). Blood collections took place on d 1 of each period, on d 43 of energy restriction and d 44 of energy repletion, and when heifers were confirmed to recommence estrous cycles. When heifers were cycling, their estrous cycles were synchronized to ensure hormone sampling occurred during late diestrus or early proestrus. Energy restriction resulted in decreased concentrations of LH (FAT P = 0.02; MOD P < 0.001), IGF-1 (FAT P < 0.001; MOD P = 0.003), and insulin (P < 0.001); in contrast, concentrations of GH (P < 0.001) and PUN (P < 0.001) increased. During repletion, LH concentration increased (P = 0.03) in MOD condition heifers but was still lower (P = 0.002) than d 1 of restriction, whereas LH concentration tended to increase in FAT heifers (P = 0.06) until it was similar (P = 0.40) to d 1 of restriction. Repletion also increased concentrations of IGF-1 (P < 0.001), insulin (P < 0.001), and glucose (P < 0.001), whereas concentrations of GH (P < 0.001), NEFA (P < 0.001), and PUN (P < 0.001) decreased. For both treatments, concentrations of GH following repletion were similar (FAT P = 0.88, MOD P = 0.10) to those on d 1 of restriction. Following repletion, FAT condition heifers had lower concentrations of IGF-1 (P < 0.001), insulin (P < 0.05), and glucose (P < 0.001), but higher concentrations of acetate (P < 0.01) and butyrate (P < 0.05), than MOD heifers. Anestrus or resumption of estrous cycles seems to be activated gradually in response to dietary manipulation, unrelated to certain metabolite changes.
J. M. Cassady, T. D. Maddock, A. DiCostanzo and G. C. Lamb. Initial body condition score affects hormone and metabolite response to nutritional restriction and repletion in yearling post-pubertal beef heifers. Published online first on April 9, 2009. J. Anim Sci. 1910. doi:10.2527/jas.2008-1716
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