Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 7, 2009
Publication Date: May 22, 2009
Citation: Roberts, A.J., Geary, T.W., Grings, E.E., Waterman, R.C., MacNeil, M.D. 2009. Reproductive performance of heifers offered ad libitum or restricted access to feed for a 140-d period after weaning. Journal of Animal Science 87:3043-3052. Interpretive Summary: Reproductive performance was evaluated in heifers born in 4 yr that were developed on either control (fed to appetite; n = 268) or restricted (fed at 80% of that consumed by controls adjusted to a common BW basis; n = 263) levels of feeding during a 140-d postweaning trial, beginning about 2 mo after weaning, at 6 mo of age. Heifers were fed a diet of 64% corn silage, 23% alfalfa and 13% of a protein-mineral supplement (dry matter basis). Restricted fed heifers consumed 27% less feed over the 140-d trial and had lower ADG (1.1 vs. 1.5 lb/d) than control heifers. After the trial, heifers were combined and subjected to an estrous synchronization protocol. Heifers were artificially inseminated at about 14 mo of age and then exposed to bulls for the remainder of a 51-d breeding season. Prior to breeding, heifers that were developed on the restricted level of feeding weighed 692 lb, whereas control fed heifers weighed 741 lb. The proportion of heifers attaining puberty by 14 mo of age was less in restricted (59%) than control fed heifers (69%). The level of restriction imposed in this study was resulted in a 20-d delay in puberty. This change in age of puberty was associated with a delay in conception of about 1 week in some, but not all years. However, overall pregnancy rate was not significantly influenced by restricted feeding of heifers. In addition, average daily gain from May to Dec, while heifers were grazing native range, was 16% greater in heifers that had been restricted than control fed heifers. Differences in market values of heifer calves and open heifers will impact potential economic advantages of restricted feeding, but an analysis utilizing average prices from the last 6 years indicated a $22 saving per bred heifer reared on the restricted level of feeding.
Technical Abstract: Reproductive performance was evaluated in composite heifers born over a 4-yr period that were randomly assigned to control (fed to appetite; n = 268) or restricted fed at 80 % of that consumed by controls adjusted to a common BW basis; n = 263) feeding for a 140-d period beginning about 2 mo after weaning, at 6 mo of age, and ending at about 12.5 mo of age. Heifers were fed a diet of 64% corn silage, 23% alfalfa, and 13% protein-mineral supplement (DM basis). Restricted heifers consumed 27% less feed over the 140 d and had lower ADG (0.49 ± 0.01 vs. 0.68 ± 0.01 kg/d; P < 0.001) than control heifers. After 140 d, all heifers were placed in common pens and subjected to an estrous synchronization protocol to facilitate artificial insemination at about 14 mo of age. Heifers were then exposed to bulls for the remainder of a 51-d breeding season. Average BW of heifers diverged within 28 d after initiation of restriction, and differences (P < 0.001) persisted through the prebreeding period (315 ± 2 vs. 337 ± 2 kg at approximately 13.5 mo of age) and subsequent grazing season (403 ± 2 vs. 414 ± 2 kg at about 19.5 mo of age). From the end of the 140 d restriction, at about 12.5 mo of age, to 19.5 mo of age ADG was greater (P < 0.001) in restricted heifers than control heifers (0.50 ± 0.01 vs. 0.42 ± 0.01 kg/d). Proportion of heifers attaining puberty by 14 m of age was less (P = 0.003) in restricted (59 ± 3 %) than control fed heifers (69 ± 3 %). To reduce bias in comparing means differing in the percentage pubertal, means of age at puberty were adjusted assuming age at puberty to be normally distributed. Adjusted age at puberty was greater in restricted heifers than control heifers (417 ± 4 vs. 397 ± 4 d; P < 0.05). Mean BW at puberty was less in restricted than control heifers. Pregnancy rates from artificial insemination were not influenced by restriction in Yr 1 and Yr 2 (P = 0.3), but were reduced by 12% in Yr 3 (P = 0.12) and 18% in Yr 4 (P = 0.05). Final pregnancy rates were 87% and 91% for restricted and control heifers respectively (P = 0.2). Economic analysis revealed a $22 reduction in cost to produce a pregnant heifer under the restricted protocol when accounting for pregnancy rates and differences in BW and market prices between selection at weaning and marketing as open heifers at ~ 1.5 yr of age. A potential economic advantage exists for rearing replacement heifers on a restricted level of feeding during the postweaning period.