Body composition and estrous cyclicity responses of heifers of distinct body conditions to energy restriction and repletion.
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Twenty Simmental x Angus, half-sibling, postpubertal heifers (initial BW of 443 +/- 9 kg) were allotted randomly to 2 treatments to evaluate if initial BCS affects how heifers respond to energy restriction and repletion. Dependent variables of interest were changes in BW, BCS, and reproductive status [determined by concentrations of serum progesterone (P(4))]. Empty body composition (EBC) was calculated using equations based on BCS. During a preliminary feeding period, diets were formulated so that each heifer in the designated treatment would reach a BCS of 5 (moderate condition; MOD) or a BCS of 7 (heavy condition; FAT). Once each heifer had reached the desired BCS, diets were formulated to supply 30% of NE(m) requirements until each heifer became anestrous (serum concentrations of P(4) < 1 ng/mL; restriction period). After anestrus, heifers were fed a high energy diet (1.43 Mcal/kg of DM) until estrous cycles resumed (serum concentrations of P(4) > 1 ng/mL; repletion period). Body weight, BCS, and EBC were determined on d 1 of each period, on d 43 of restriction, and d 44 of repletion, and when heifers were confirmed to have resumed estrous cycles (2 normal estrous cycles determined by P(4) > 1 ng/mL). Regression of BCS on BW determined individual BCS at anestrus and estrus. After 43 d of restriction, FAT condition heifers were heavier (P < 0.001), had greater BCS (P < 0.001), and had a greater proportion of empty body fat (P < 0.001). Heifers in FAT condition remained cyclic longer (P < 0.001) than those in MOD condition (148 vs. 61 d). In contrast, at the onset of anestrus, BW (P = 0.15), BCS (P = 0.54), and empty body fat were similar (P = 0.54) between treatments. At 44 d of repletion, BW (P = 0.46), BCS (P = 0.41), and empty body fat (P = 0.41) were similar between treatments. Heifers in both treatments recommenced estrous activity after similar (P = 0.43) number of days (54 d) of energy repletion, but near onset of estrous cycles, heifers in FAT condition were heavier (P = 0.002) and had greater BCS (P = 0.03) and empty body fat (P = 0.01) than those in MOD condition. Initial BCS influenced days to anestrus, but not BCS or EBC at onset of anestrus. Initial BCS had no effect on days to recommencement of estrous cycles, but did influence the degree of fatness required to resume estrous cycles.
