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Effects of maintenance energy requirements of gestating beef cows on calf performance
Spring calving, Angus cows (n = 123) were used to evaluate the effects of maintenance energy requirements (MR) of cows on calf performance and pregnancy rate of cows. In 5 replications, gestating cows (BW: 552 ± 3 kg, BCS: 4.6 ± 0.1, 7.0 ± 0.1 yr of age, 7 to 8 mo. of gestation) were individually fed a complete diet for 30 d (based on Model 1, NRC 2000). Body weights were obtained twice weekly and daily feed intake was adjusted every 2 wk until constant BW was achieved (regression analysis). Mean MR of cows was 86.6 ± 0.9 kcal*kg BW -0.75*d-1. Maintenance energy requirements were used to classify cows as low (L; < 0.5 SD less than yearly mean), moderate (M; ± 0.5 SD of yearly mean) or high (H; > 0.5 SD more than yearly mean). Body weights of calves, after exposure of cows to maintenance diets, were obtained at birth and weaning (WW). Pregnancy of cows was determined by ultrasonography at 31 ± 1 d after AI. Birth weight and 205 d WW were analyzed with PROC MIXED (SAS Inst. Inc.) with year as a covariate. Pregnancy rate was analyzed with Chi-square (PROC FREQ; SAS Inst. Inc.). Birth weights of calves did not differ among MR classification (P = 0.75). Weaning weights of calves did not differ between H (P = 0.32; 225.5 ± 4.4 kg), M (234.2 ± 3.6 kg), and L (230.2 ± 3.8 kg) cows. Steer calves had greater WW (P = 0.004; 236.5 ± 2.9 kg) compared with heifer calves (223.3 ± 3.5 kg). Pregnancy rate to first service AI did not differ (P = 0.95) between H (71%), M (71%), and L (74%) cows and pregnancy rates were similar across years (P= 0.29). These results indicate that MR of gestating cows does not influence reproduction or preweaning calf performance. Identification of cows that require less energy input while maintaining performance may improve production efficiency of cows.
Keywords: beef cattle, maintenance, calf performance