Winter grazing of stockpiled native forages during heifer development delays body weight gain without influencing final pregnancy rates

摘要

The objective of this study was to test the effects of protein supplementation strategy and different stockpiled forage species on growth, nutritional status, and reproductive performance of yearling beef heifers. In a 5-yr study, yearling beef heifers (n = 266) were stratified by body weight (BW) at weaning to 1 of 3 stockpiled forages: 1) endophyte-infected tall fescue (TF, Schedonorus arundinaceus (Schreb.) Dumort; 7.21% crude protein [CP] and 67.13% neutral detergent fiber [NDF], dry matter [DM] basis), 2) big bluestem (Andropogon gerardi Vitman) and indiangrass (Sorghastrum nutans L.) combination (BI; 4.32% CP and 71.06% NDF, DM basis), or 3) switchgrass (SG, Panicum virgatum L.; 3.87% CP and 76.79% NDF, DM basis). Forage treatments were then randomly assigned to receive 1 of 2 supplement types: 1) 0.68 kg heifer(-1) d(-1) of dried distillers grains with solubles (DDGS: 28% CP and 108% total digestible nutrients [TDN]) or 2) 0.22 kg heifer(-1) d(-1) of blood meal and fish meal (BF: 72.5% CP and 77.5% TDN), resulting in a 3 x 2 factorial arrangement of treatments. Each year, twenty-one 1.2-ha pastures (7 pastures per forage type) were utilized with 2 to 3 heifers per pastures. Treatments were initiated in January and terminated in April at the initiation of breeding. Initial BW was not different (P = 0.22) by forage or supplement type. During the rest of the grazing period, BW was greater (P 0.01) for TF heifers. However, average daily gain (ADG) was greater (P 0.01) for BI and SG heifers from breeding to final pregnancy diagnosis. Heifers grazing TF pastures had greater (P 0.01) overall ADG than their counterparts. The percentage of mature BW (MBW) at breeding was greater (P 0.01) for TF heifers. Heifer BW and ADG was not influenced (P = 0.06) by supplementation strategy. Serum glucose concentrations were not different (P = 0.44) among forage type or supplement strategy. Pregnancy rates at fixed timed-artificial insemination and overall pregnancy rates did not differ (P = 0.38) by forage or supplement treatment. Owing to forage nutritive value differences, heifers grazing low-quality, warm season grasses lost BW prior to the initiation of the breeding season. However, a negative BW gain prior to breeding did not negatively impact overall pregnancy rates.