Changes in body composition during winter gestation of mature beef cows grazing different herbage allowances of native pastures.
The aim of this study was to evaluate the effect of controlling the intensity of grazing native pasture (Campos biome), through control of herbage allowance (HA), on body composition (water, protein and fat) of beef cows of different genotype during the winter gestation period. Mature beef cows (n=32) were used in a complete randomized block design with a factorial arrangement of HA (2.5 vs. 4 kg DM/d; LO vs. HI) of native pastures (52% DM, 8.4% crude protein, 39.7% acid detergent fiber) and cow genotype (CG; Angus-Hereford vs. F1 reciprocal crosses; PU vs. CR). The experiment was conducted during three years and at the end of the third year at 150, 210 and 240±10 days of gestation (during winter) and 190±10 days pospartum (fall), body composition was estimated using the urea dilution technique. In addition, at 192±10 days postpartum cows were slaughtered and weight and samples of all tissues and organs were collected for chemical composition analyses. Multiple regressions using urea space volume, and other animal characteristics as predictors, were adjusted by the regression procedure (SAS Institute Inc.) using data obtained at slaughter to estimate body components (kg) during the winter gestation period. Data of body composition during gestation were assessed using a mixed model repeated measures analysis. During the winter gestation period, maternal live weight (LW) and BCS tended (P=0.09) or were greater (P=0.02) in HI than LO cows and in CR than PU cows. Maternal LW and BCS decreased (P<0.05) 18 ± 5 kg and 0.5 ± 0.1 units from 150 to 210 days of gestation. Total body water, fat and protein mass were greater (P≤0.05) in CR than PU cows. Body fat and protein mass tended (P≤0.10) to be greater in HI than LO cows. Total body water and protein mass decreased (P<0.01) from 150 to 210 days of gestation. Body water was affected (P=0.03) and body fat tended (P=0.07) to be affected by the interaction between CG and days of gestation as they were greater (P<0.05) in CR than PU cows only at 150 days of gestation and decreased from 150 to 210 days of gestation only in the former ones. Body composition during winter gestation depended on both, HA and CG. Changes in the composition of body weight lost or retained would influence energy maintenance requirements and could provide metabolic advantages under periods of negative energy balance.
Keywords: fat, protein, cattle, rangelands