Evaluation of biological efficiency of free-grazing beef cows under semidesert conditions.

Effects of cow BW, hip height, and estimated genetic potentials (EBV) for weaning weight direct and milk on cow productivity, fecal OM output, OM intake, and efficiency (kilograms of calf BW/kilogram of OM intake by the cow) were evaluated with 44 free-grazing crossbred cows under semidesert conditions. Calf BW were measured during early, mid-, and late lactation. Data were collected in four periods: Period 1 = late spring (early lactation), Period 2 = late summer (mid-lactation), Period 3 = mid-autumn (late lactation), and Period 4 = mid-winter (nonlactation). Calf BW increased linearly with cow BW (P < .01) in Periods 1, 2, and 3. Fecal OM output and OM intake increased with cow BW in Periods 2 (P < .01) and 4 (P < .01), and on average (P < .02). Overall efficiency decreased with increasing cow BW (P < .04). Taller cows excreted more fecal OM and had greater OM intake throughout the study (P < .02 to P < .11). Overall efficiency decreased with increasing cow hip height (P < .05). Weaning weight direct EBV of cows was related linearly to cow BW (P < .01 to P < .07) and to calf BW (P < .01 to P < .07). Calf weight in all periods increased linearly with milk EBV (P < .001). Overall, fecal OM output, OM intake, and efficiency were not affected by milk EBV.