The effect of shifting calf-weaning age on profiles of energy status (BW, BCS, and rib and rump fat) and reproductive performance of beef cows was evaluated in a 3-yr study. Pregnant and lactating crossbred beef cows (n = 408), mainly of Angus and Hereford breeding, were stratified by age and by sex and BW of their calves and assigned randomly into 2 treatments: weaning at approximately 180 d (early weaning) and normal weaning 45 d later (control). Cows were managed together on native range pastures and supplemented with harvested forage during the winter months. Cow BW, BCS, rib fat, and rump fat were measured periodically from early weaning through the next breeding. Reproductive performance was evaluated by calving intervals (CI), days from initiation of breeding to calving (BCI), retention in the herd, and adjusted 205-d weaning BW of the subsequent calf. Early weaned cows had greater (P < 0.001) BW at normal weaning than control cows, but the overall pattern of cow BW did not differ (P > 0.05) among treatments. Peak and nadir BCS occurred at precalving and postcalving periods, respectively and were greater (P < 0.001) at each period in early weaned than in control cows and in cows > or =5-yr-old than in younger cows. Patterns for rib fat and rump fat were nearly identical to those of BCS except for the 3-way interaction (P < 0.001) of treatment, age, and period on rump fat. Mean CI (372.4 +/- 2.1 d) and BCI (299.7 +/- 1.9 d) were not affected (P = 0.42) by treatment but varied (P < 0.001) with age of the cow. Age of cow accounted for 16% of total variation in CI and 12% of total variation in gestation length (P < 0.001). The intervals were longer (P < 0.001) in primiparous cows than in older cows. Early weaning decreased risk of culling in cows and thereby increased (P < 0.05) overall persistence by 11% over control cows. Earlier weaning of cows in the previous year increased (P < 0.001) weaning weight of the subsequent calf by 8.6 kg per cow per yr. Shifting weaning time increased storage of consumed energy as evidenced by increased rump fat, for use later during high-energy demand, ultimately improving overall productivity of the cow-calf system.

Preconditioned calves have greater market value per unit weight than normal-weaned calves. Development of a low cost forage-based preconditioning system allows producers to add value to their calf-crop. This study evaluated calf performance in three forage-based weaning systems; early-weaned calves were backgrounded in legume/grass forage plots and supplemented with commercial preconditioning feed (Treatment 1) or an on-farm corn-mix (Treatment 2). Control (Treatment 3) calves suckled for an additional 45 days. Supplements provided 2.17 kg TDN/calf/ day. Weights were collected on days 30, 0 and 45 with respect to early weaning, from 135 in year 1 and 150 calves in each of the two subsequent years. Effects of treatment, age of dam, sex of calf and their interactions on calf weight gain were analyzed by analysis of covariance using GLM procedures of SAS. Marginal effects of treatment and feed cost were used to evaluate economic feasibility. Sensitivity analyses were evaluated for anticipated market fluctuations in feed costs and calf premiums. Data are reported as least squares means. Calf weight gains differed (P < 0.001) among treatments and averaged 1.16, 1.03 and 1.04 kg/calf/day for commercial supplement, corn-mix and controls, respectively. Calves from 2-year-old cows gained less (P < 0.001) weight compared to those from cows 3 -4 and ≥5 years of age (44.8, 48.9 and 51.5 kg, respectively). Steers calves gained more (P < 0.001) weight compared to heifer calves (51.2 vs. 45.7 kg, respectively). Net returns for corn mix were greater than those for commercial feed ($1.48 vs. $1.35/kg weight gain, respectively). Sensitivity analyses indicated that selection of preconditioning treatment to a large degree was less sensitive to significant changes in market conditions due to the large gap in marginal costs between the two treatments. In conclusion, forage-based weaning systems can be utilized to precondition calves providing an economical means for calf weight gain and profit potential as long as feed costs are held within reasonable limits.