Beef heifer growth and reproductive performance responses following two levels of fall stockpiled forage allocation.

摘要

Recent research on heifer development systems has been conducted primarily in dry-lot settings and limited information exists comparing systems which utilize standing forage (Larson and Funston, 2011). Objectives of the research presented here include: 1) comparing heifer growth and reproductive performance in response to two levels of stockpiled fall forage allocation: daily herbage DM allocation of 3.5 (LO) or 7.0 (HI) % of BW; and 2) assessing the seasonal dynamics of forage quality, herbage mass, and botanical composition of naturalized pastures containing mixed cool-season species in response to two different fall stocking rates (HI vs LO). Treatments were replicated 3 times per year for 3 years (2009-2012). There was a significant treatment x year (P < 0.01) and year x sampling period interaction (P < 0.05) for herbage mass. In years 1 and 2, herbage mass declined steadily in both HI and LO treatments. From early in the fall grazing period to the middle of the period herbage mass declined 9.1% and 8.6% (year 1) and 10.4% and 7.0% (year 2) for LO and HI treatments, respectively. From the middle of the fall grazing period to late in the period, herbage mass declined 10.0% and 8.5% (year 1) and 9.7% and 8.9% (year 2) for LO and HI treatments, respectively. In year 3, however, herbage mass declined at a much faster rate than in years 1 and 2 from early in the fall period to the middle portion of the period (15.2% for both LO and HI treatments). This rapid decline in year 3 is most likely due to the high percentage of legumes present. There was no treatment effect, but there was a significant year effect (P = 0.05) for CP % during the fall grazing period. Mean CP % was 15.8% (year 1), 19.4% (year 2,) and 17.0% (year 3). Throughout the fall grazing period, CP declined in both treatments from 18.3% (early grazing) to 18.1 % (mid grazing) and 16.2% (late grazing) (P = 0.003). There was a significant year effect (P < 0.001) and there tended to be a treatment effect (P = 0.06) for NDF. There was a significant year x sampling period interaction (P = 0.009) for IVTDMD and it declined throughout the fall grazing period, 81.9%, 79.4%, and 75.4% for early, middle, and late portions of the grazing period. Nutritive content of the pastures was adequate to meet the requirements of beef heifers. Heifers in the LO group gained 0.12 kg/d whereas heifers in the HI group gained 0.40 kg/d during the fall grazing period (P < 0.0001). Fall ADG was affected by NDF content of the pasture; for each 1 percentage unit increase in NDF, fall ADG decreased 0.14 kg (P < 0.05). During winter feeding, ADG was 0.30 kg/d and 0.39 kg/d for LO vs HI heifers, respectively (P < 0.001). During the spring grazing period, LO heifers had numerically higher ADG than HI heifers (1.38 vs. 1.30 kg/d; P = 0.64). Hip height (122.7 vs. 121.4 cm; P < 0.01), BCS (5.8 vs 5.6; P < 0.01), and BW (356 vs. 335 kg; P < 0.0001) at the end of spring grazing was higher for HI heifers than LO heifers. Heifers in the LO group compensated with greater summer ADG than heifers in the HI group (0.74 vs. 0.66 kg/d; P < 0.05). Total ADG from treatment initiation (November) through pregnancy diagnosis (August) was higher for HI heifers than LO heifers (0.61 vs. 0.55 kg/d; P < 0.001) as was BW at pregnancy diagnosis (415 vs. 402 kg; P < 0.01). Percentage of heifers reaching puberty by the time of AI was 34% for both groups (P = 0.93). Percentage of heifers becoming pregnant to AI tended (P = 0.13) to be higher for HI heifers (44%) than for LO heifers (32%). Fall ADG across treatment groups affected the probability of a heifer becoming pregnant by AI (P < 0.05). The probability of a heifer becoming pregnant by AI with ADG in the fall of 0 kg, 0.5 kg, and 1.0 kg is 29%, 46%, and 64%, respectively. Percentage pregnant to the bull (61% for LO vs. 59% for HI; P = 0.80) and final pregnancy rate (74% for LO vs. 77% for HI; P = 0.61) was similar for the two groups. We interpret these results to indicate that: 1) mixed cool-season naturalized pastures can be effectively stockpiled for fall and winter grazing; and 2) delaying the majority of weight gain until late in heifer development can decrease costs of winter feeding and potentially result in adequate overall pregnancy rates.