Beef heifer development within three calving systems

¹ USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301

^* To whom correspondence should be addressed. E-mail: elaine.grings{at}ars.usda.gov.

	Abstract

A 3-yr study was conducted to evaluate impacts of calving system, weaning age, and post-weaning management on growth and reproduction in beef heifers. Heifer calves (n = 676) born in late winter (LW; average birth date = Feb 7 ± 9 d) or early spring (ES; average birth date Apr 3 ± 10 d) were weaned at 190 or 240 d of age and heifers born in late spring (LS; average birth date May 29 ± 10 d) were weaned at 140 or 190 d of age. Heifers were managed to be first exposed for breeding at approximately 14 mo of age. After weaning calves were randomly assigned to treatments. Heifers on the constant gain (CG) treatment were fed a corn silage and hay-based diet. Heifers on delayed gain (DG) treatments were placed on pasture but were fed grass hay and/or a supplement depending on forage conditions. Three months before their respective breeding seasons, DG heifers were moved to drylot and fed a corn silage and barley-based diet (LW or ES) or moved to spring rangeland (LS). Data were analyzed using mixed model procedures with calving system, weaning age, and post-weaning management options creating 12 treatments. Year was considered a random effect. Non-orthogonal contrasts were used to make treatment comparisons. Average daily gain was 0.36 ± 0.05 (SED) kg/d less (P < 0.001) for DG heifers during the initial phase, whereas these heifers gained 0.44 ± 0.03 kg/d more (P < 0.001) than CG heifers during the last 90 d before breeding. Weights at the beginning of the breeding season did not differ (P = 0.97) between CG and DG heifers, but were affected by calving system and weaning age, reflecting some of the differences in initial BW. Pre-breeding BW for heifers weaned at 190 d were 36 ± 6.4 kg heavier (P < 0.001) for heifers born in LW and ES compared with LS and were 388, 372, and 330 kg for heifers weaned in Oct at 240, 190, or 140 d of age (linear effect, P < 0.001). Proportion of heifers exhibiting luteal activity at the beginning of the breeding season was not affected (P = 0.57) by treatment. About half of the heifers were randomly selected for breeding. Treatment had no effect (P = 0.64) on pregnancy rates. In conclusion, heifers from varied calving systems and weaning strategies can be raised to breeding using either constant or delayed gain strategies without affecting percentage of heifers cycling at the beginning of the breeding season. These results suggest that producers have multiple options for management of heifer calves from differing calving systems.

Key Words: beef heifers, calving date, weaning

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