Submitted to: Western Section of Animal Science Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2012
Publication Date: 7/15/2012
Citation: Mulliniks, J.T., Hawkins, D.E., Kane, K.K., Cox, S.H., Torell, L.A., Scholljegerdes, E.J., Petersen, M.K. 2012. Metabolizable protein supply alters pregnancy and subsequent retention rate during heifer development while grazing dormant winter forage. Western Section of Animal Science Proceedings 63:306-310. Interpretive Summary: Selection and development method of replacement heifers can impact future productivity and longevity of the entire cowherd. Heifers developed on native dormant range fed 0.9 kg/d of a 36% CP supplement containing 50% ruminally undegradable protein prior to breeding may increase in herd retention rate and productivity with an overall lower cost of development. This study demonstrates that environmentally adapted heifers can be grown at a slow rate of gain on semi-arid rangelands with strategic supplementation, resulting in pregnancy rates similar to heifers in a moderate to high rate of gain, while improving future productivity. Developing heifers on native range and supplementing with RUP has the potential to increase retention rates based on greater reproductive success.
Technical Abstract: Type of heifer development system can have major impact on the future productivity and retention rate of the cowherd. Therefore, the objective of this experiment was to determine growth, reproductive performance, retention rate, and economic efficiency of heifer’s developed in a range raised (with 2 CP treatments) or high input (feedlot developed) heifer development system. Spring-born, crossbred heifers (n = 191) were stratified to 1 of 3 treatments at weaning: (1) 0.9 kg/d of a 36% CP supplement containing 64% RDP (CSM), (2) 0.9 kg/d of a 36% CP supplement containing 50% RDP (RUP), or (3) a concentrate diet fed in dry lot (CONC) to gain 0.68 kg/d. Supplementation was initiated in February and terminated at the onset of a 45-d breeding season in May. Heifer BW and hip height (HH) were taken monthly from initiation of supplementation until breeding and again at weaning. Females were removed from the herd for failure to reproduce or wean a calf. Percent of heifers becoming pregnant and remaining in the herd at start of each breeding season was recorded to determine retention rate throughout their 3rd calf crop. Breeding BW was greater (P < 0.01) for the CONC than CSM or RUP developed heifers. Hip height at breeding was greater (P < 0.01) in RUP and CSM relative to CONC heifers. However, palpation BW and HH was similar (P > 0.24) for all heifer development treatments. At breeding, RUP and CSM heifers reached 51% of mature BW (P < 0.01) compared to CONC heifers at 58% of mature BW. Pregnancy rates were 94, 88, and 84% for RUP, CSM and CONC heifers (P = 0.10). Net return was $99.71 and $87.18 per developed heifer greater for RUP and CSM heifers, respectively compared to CONC heifers due to differences in pregnancy and development costs. Retention rate at age 4 was greatest (P < 0.01) for RUP heifers. This study indicates that range developed heifers can be as reproductively successful as heifers developed in a feedlot, while improving future productivity. Furthermore, metabolizable protein supply improves reproduction in heifers developed on dormant native range.