Effects of dietary protein level on fattening of cull Charolais cows

Abstract

Two experiments involving 82 cows have been arried out to evaluate the effects of dietary protein level for finishing cull Charolais cows. The objectives of these studies were to determine the effects of providing supplementary protein in the form of soybean meal to cull cows fed maize silage. The daily gain, feed efficiency, carcass traits, composition of gain and physical and chemical characteristics of longissimus dorsi and rhomboideus thoracis muscles were evaluated. In each trial, forty-one thin, non-pregnant cows averaging 5 years old were divided into five groups according to body condition, age and live weight. A group of five cows were slaughtered prior to the feeding trial in order to determine initial carcass composition. Three groups (N = normal, M = medium, H1 = high) received diets containing 90, 120, 160 g of PDI per UFL. Maize silage was offered ad libitum to N and the intake of groups N and Hl was ajusted on the intake of N to make the three diets iso-energetic. The fourth group (H2) was offered maize silage ad libitum, supplemented with 160 g of PDI per UFL like H1 to evaluate the effect of high level protein on silage intake. The cows of group N were slaughtered at an optimal body condition visually estimated (score 3,5 out of 5) and the other three groups at the same average length of feeding time as group 1 (81 days for trial 1 and 75 days for trial 2). ). With the similar energy intake (N, M, H1) liveweight gains were not significantly increased by the protein concentration of the diet (1320 g/d for H1 vs 1070 g/d for N) and the carcass weights did not differ (376 kg and 382 kg for groups N et H1 respectively). The proportion of fat in the carcass and the internal fat tended to decrease with the higher protein level. Meat characteristics as shear measurements, water holding and lipid, collagen, iron pigment contents were not influenced by protein level in the diet. When maize silage was fed ad libitum with the higher protein level, daily silage dry matter intakes increased by 0,7 kg (0,6 UFL). This increasing energy intake resulted in positive but not significant weight gain response and in an increase of fat in the carcass and in the fifth quarter.