Changes in body composition during the winter gestation period in mature beef cows grazing different herbage allowances of native grasslands

Abstract

The aim of this study was to evaluate the effect of controlling the grazing intensity of native pastures, through the herbage allowances (HA) on body composition (water, protein, and fat) of beef cows of different cow genotype (CG; purebred: Angus and Hereford; PU, and crossbred: reciprocal F1; CR). Mature beef cows (n = 32) were used in a complete randomised block design with a factorial arrangement of HA (2.5 vs 4 kg DM/day; LO vs HI) and CG (PU vs CR). The experiment was conducted during 3 years and at the end of the third year at 150, 210, and 240 ± 10 days of gestation and 190 ± 10 days postpartum body composition was estimated using the urea dilution technique. At 192 ± 10 days postpartum cows were slaughtered and all tissues and organs were weighed and samples were collected for chemical composition analyses. During the winter gestation period, body condition score was greater (P < 0.05) in HI than LO cows and in CR than PU cows. Relative body water (g/kg of empty bodyweight) was greater (P ≤ 0.02) in HI than LO cows and in CR than PU cows, whereas relative body protein did not differ between HI and LO, but tended (P = 0.10) to be greater in CR than PU cows. In contrast, relative body fat tended (P = 0.10) to be greater and gross energy content was greater (P < 0.01) in HI than LO cows, whereas they did not differ between CR and PU cows. Relative body water increased (P < 0.01) from 150 to 210 days of gestation for all cows whereas relative body fat decreased (P < 0.05) and body protein increased (P < 0.05) from 150 to 210 days of gestation in PU but not in CR cows. These results suggest that HI maintained greater body condition score and retained gross energy content when compared with LO cows, and CG affected not only body composition (greater body fat and protein in CR than PU cows) but also composition of mobilised/retained weight during the winter gestation period with a greater protein tissue mobilisation in CR than PU cows.