|Paterson, J - MONTANA STATE UNIVERSITY|
|Lipsey, - AMERICAN SIMMENTAL ASSO|
Submitted to: Livestock Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 30, 2012
Publication Date: June 13, 2012
Citation: Waterman, R.C., Geary, T.W., Paterson, J.A., Lipsey 2012. Early weaning in Northern Great Plains beef cattle production systems: II. Development of replacement heifers. Livestock Science 148:36-45. Interpretive Summary: Sustainability of extensive beef production enterprises that develop replacement females hinges on reproductive success of those females when exposed to breeding by 450 d of age. Proper development of replacement heifers is crucial in order to obtain puberty and promote lifetime productivity and optimal milking ability. Studies have demonstrated that calves born and weaned in different seasons can influence body weight at weaning. Further, studies have concluded that heifers, when weaned at 140, 190, or 240 d of age, and subsequently developed on either constant or delayed gain management strategies can have similar reproductive success. No differences in attainment of puberty by a given date were observed for spring–born heifers weaned on fall pasture followed by a period of drylot confinement when compared to heifers consuming a corn silage based diet throughout the same period in the Northern Great Plains. Previous research has concluded that producers have many options to develop heifers as long as necessary weight is achieved by breeding. The objectives of this study were to evaluate the effect of weaning heifer calves early (approx. 80-d) or at normal (approx. 215-d) weaning age, on BW gain to 215-d, subsequent development BW gain, luteal activity, and pregnancy rate in two herds located in the Northern Great Plains. In conclusion, changes were observed for BW and serum metabolites for both the weaning and heifer development portions of this study. However, it is important to point out that puberty and reproductive success for heifers were not compromised by weaning treatments. Heifers became pubertal prior to synchronization, bred by 15 mo of age, and conceived to AI or natural mating. All three experiments demonstrate that both early and normal weaned heifers can be developed and bred in extensive beef production systems in the Northern Great Plains. In agreement with other studies timing in which gain occurs is likely less important than achieving an optimal BW by time of breeding. Therefore, weaning heifer calves early in times where forage availability may be limited or young cows need greater condition going into winter will not impact the opportunity to retain replacement females that can replenish a cow herd.
Technical Abstract: A 2-yr study was conducted to evaluate effects of weaning heifer calves early (approx. 80-d) or at the normal (approx. 215-d) weaning age on BW gain to 215-d, subsequent BW gain, luteal activity, and pregnancy rate in two herds located in the Northern Great Plains. In exp. 1 and 2, heifer calves from crossbred dams (predominantly Angus (= 75%) with the remainder beign Hereford, Red Angus, Charolais, and Tarentaise at the USDA-ARS, Fort Keogh Livestock and Range Research Laboratory (LARRL) were stratified within dam age and calving date. Heifer calves were randomly assigned within strata to one of three weaning treatments. At approximately 215 d of age, all heifers were gathered and brought into lots at the LARRL. Heifers were then stratified by weaning treatment into two heifer development diets. In exp. 3, at Judith Gap, MT, heifer calves from Angus and Angus × Simmental dams were stratified within breed group by age, calving date, and AI sire and were assigned within strata to one of three weaning treatments at the start of breeding. At approximately 215 d of age, all heifers were gathered and brought into lots and commonly fed a development diet. Heifers from 2-yr-old dams that were early weaned weighed more at 215 d (P < 0.001; exp. 1 and 2), and all early weaned heifers in exp. 3 weighed 17 kg more than normal weaned heifers. Body weight gain through the heifer development period was not affected by diet (P > 0.05). Age at attainment of puberty was similar in exp. 1 and 2 (P > 0.05). Pregnancy rates were not influenced by early weaning or heifer development treatments (P > 0.05). Pregnancy rates were not compromised by early weaning heifer calves at 80 d of age and attainment of puberty occurred prior to estrous synchronization in exp. 1 and 2. Puberty was not assessed in exp. 3. All three experiments demonstrate early weaning as viable option to develop and breed heifers in extensive beef production systems in the Northern Great Plains. Weaning heifer calves early in times when forage availability may be limited or young cows need greater condition going into winter will not impact the opportunity to retain replacement females.