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

Two experiments were conducted in which growing sheep were given choices be tween foods differing in their contents of urea. The crude protein (CP) con tents of both of the foods of the pair were also varied. The hypothesis tes ted was that the diets selected would be those that met the effective rumen degradable protein (eRDP) requirements of the animals and would avoid exce ss 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 fo ods to make a further six foods. In both experiments Texel X Greyface femal e sheep were used. In experiment 1, 34 sheep weighing 37.2 (s.d. 1.85) kg w ere randomly allocated to one of six groups and each group was offered a ch oice between a pair of foods. Groups 1 to 3 were offered pairs in the D ser ies (D v. D + 12.5 g urea per kg (no. = 6), D v. 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 exp eriment 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 experimen t. Groups 7 to 12 were given a choice (no. = 10) between two foods. One foo d of the pair was the basal D, H or P. The other food was the same basal fo od supplemented with either 12.5 or 25 g urea per kg. On the single foods a dding 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 n o beneficial effects on intake or liveweight gain. This suggests that both contained sufficient eRDP in relation to energy. Across all choice treatmen ts 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 experi ment 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 food s used. The results do not support the hypothesis that sheep will avoid exc ess eRDP when given a choice and suggest that eRDP may not be a relevant di mension in diet selection in the conditions of these experiments.