Diet selection of sheep: effects of adding urea to foods with different protein contents

Abstract

AbstractTwo experiments were conducted in which growing sheep were given choices between foods differing in their contents of urea. The crude protein (CP) contents of both of the foods of the pair were also varied. The hypothesis tested was that the diets selected would be those that met the effective rumen degradable protein (eRDP) requirements of the animals and would avoid excess consumption of eRDP. Three basal foods, differing in CP were formulated. D was designed to be deficient in CP with 80 g CP and 58 g eRDP per kg DM; H was calculated to be adequate with 159 g CP and 114 g eRDP per kg DM; P had excess with 210 g CP and 148 g eRDP per kg DM. Other foods were made by adding 12·5 or 25 g urea per kg fresh matter to each of the three basal foods to make a further six foods. In both experiments Texel✕Greyface female sheep were used. In experiment 1, 34 sheep weighing 37·2 (s.d. 1·85) kg were randomly allocated to one of six groups and each group was offered a choice between a pair of foods. Groups 1 to 3 were offered pairs in the D series (Dv. D + 12·5 g urea per kg (no. = 6), Dv. D + 25 g urea per kg (no. = 6) and D + 12·5 g urea per kg v. D + 25 g urea per kg (no. = 5)). Groups 4 to 6 were offered the same pairs of foods but with H instead of D. In experiment 2, 96 sheep weighing 29·8 (s.d. 3·37) kg were randomly allocated to one of 12 groups. Groups 1 to 6 (no. = 6) were allocated a single food (D, D + 25 g, H, H + 25 g, P or P + 25 g urea per kg) throughout the experiment. Groups 7 to 12 were given a choice (no. = 10) between two foods. One food of the pair was the basal D, H or P. The other food was the same basal food supplemented with either 12·5 or 25 g urea per kg. On the single foods adding urea to D resulted in an increase in food intake and live-weight gain suggesting that D was deficient in eRDP. Adding urea to either H or P had no beneficial effects on intake or live-weight gain. This suggests that both contained sufficient eRDP in relation to energy. Across all choice treatments in both experiments there was a highly significant preference (P< 0·01) for the food with the higher urea content. In experiment 1 0·62 (s.e. 0·04) of the diet selected was the food with the higher urea content. In experiment 2 the figure was 0·64 (s.e.0·03). The general preference for the food with the higher urea content was unaffected by the CP contents of the foods used. The results do not support the hypothesis that sheep will avoid excess eRDP when given a choice and suggest that eRDP may not be a relevant dimension in diet selection in the conditions of these experiments.