The effects of three levels of crude protein in the diet, 10, 12 and 14 %, on the performance of entire Israeli Friesian male cattle (bulls) during fattening, was studied in two experiments.
The bulls were 5 months old and 200 kg live weight, and 7·5 months and 250 kg, at the start of Experiments 1 and 2, respectively, and were slaughtered after 178 to 268 days (Experiment 1) or 156 to 166 days (Experiment 2) on trial, at a live weight of 430 to 480 kg.
In Experiment 1 mixed diets of two metabolizable energy concentrations (11·1 and 10·5 MJ metabolizable energy/kg dry matter) were used, each containing concentrates with 10, 12 or 14% crude protein content. In half of the treatments the crude protein content of the concentrate was increased by two percentage units for the first 61 days of the experiment. Daily live-weight gain was significantly higher at 14% and 16% crude protein than at 10% and 12% crude protein during the first 61 days for animals on the high energy diet, but liveweight gain was not affected by crude protein content on the low energy diet over this period, or at either energy concentration from 62 days to slaughter.
In Experiment 2 the diet consisted of concentrate, wheat straw and cotton hulls, giving a metabolizable energy concentration of approximately 10·55 MJ/kg dry matter. Daily live-weight gain did not differ significantly between concentrates containing 10, 12 or 14% crude protein, but carcass weight gain was higher at 12% and 14% crude protein than at 10% crude protein.
It is concluded that in order to ensure the best performance of bulls of the Israeli Friesian breed, diets with a high concentration of energy should contain 14% crude protein when fed to animals with a body weight of 200 to 300 kg, and 12% crude protein for heavier animals.