Effect of protein supply on the performance of intensively reared bulls - evaluation of the DCP and the Nordic AAT-PBV protein evaluation systems

Effects of level and origin of protein supplements on feed intake, live weight gain and efficiency of feed utilization of intensively reared bulls were studied in five experiments, (A) to (E). Experimental periods comprised live weights (LW) from 80 to 500 kg and treatment averages for daily live weight gain (LWG) ranged from 1.00 to 1.40 kg. Concentrates were fed ad lib. in all experiments, except in (B) and (E) where feed intake was restricted for half or for all of the animals, respectively. In concentrates, based on oats and barley, soyabean meal (A, B), peas, rapeseed meal or soyabean meal (C), soyabean meal and fish meal (D) or coconut cake, fish meal and urea (E) were used to alter the protein content. The effects of protein supply were evaluated in terms of the Nordic MTPBV system. In the data compiled for all experiments the calculated amount of amino acid crude protein absorbed in the small intestine (MT) per MJ .ME intake varied between 6.7 to 7.7 g in the LW range 100 to 200 kg. The efficiency of .ME utilization was improved in all experiments when the MT content per MJ .ME increased. A diminishing return in efficiency of ME utilization was observed in (A) at the highest MT content per MJ ME (7.5 g). This was interpreted as an effect of a simultaneously high value for the protein balance in the rumen (PBV) (3.1 g per MJ ME) rather than of the MT supply per se. Also low PBV values (less than -2 g per MJ ME) seemed to reduce efficiency of .ME utilization. For LW above 200 kg, efficiency of ME utilization was not affected when the MT content per MJ .ME increased from 6.7 to 7.4 g. For rations with PBV values less than -2 g and higher than 2 g per MJ ME the MT and PBV contents together provide a better explanation of the differences in performance than the digestible crude protein content of the ration. Replacing soyabean meal (9 % of the concentrate) with conventional high glucosinolate rapeseed meal (12 %) did not affect LWG or efficiency of feed utilization, but a 50 % increase in the weight of the thyroid glands was observed. When soyabean meal was replaced by peas (25 % of the concentrate) an increased thyroid weight was also found, although the enlargement was less (25 %). Except for a high fat deposition at slaughter when large amounts of peas were included in the concentrate, no effects of protein supply on fat deposition or carcass composition were found.