Urea supplementation of roughage-based diets for cattle

Abstract

SUMMARYThirty-two growing bullocks were fed long barley straw ad libitum in each of three experiments; urea was added to the latter or to a diet of straw supplemented with either 1·36 or 2·72 kg meal. The amount of urea fed was 75 g per animal daily. In a fourth experiment, a basal allowance of 3·64 kg chopped hay was fed to thirty-two bullocks weighing 287 kg; in addition 4·55 kg of meal was fed in which none, one-half or all the supplemental nitrogen was supplied as urea. Rate of gain and straw intake of animals given straw only was similar to that of animals given straw and urea. Feeding 1·36 kg of either rolled barley or rolled barley supplemented with 10% soyabean meal, in addition to straw, did not elicit a response from urea. A response was obtained from urea when 2·72 kg of a barley–soyabean meal mixture was fed with straw. Blood urea concentrations generally reached a maximum at the third week of urea feeding and declined thereafter. Addition of urea to straw raised blood urea to about the same extent as incorporating urea with 1·36 kg of rolled barley. Blood concentrations were higher when the urea was fed with meal than when mixed with molasses. Nitrogen retention from urea was not significantly increased by feeding 1·36 kg of barley with straw ad libitum as compared to the retention on an all-straw ration. Slightly more nitrogen was retained when urea was added to rolled barley instead of a rolled barley–soyabean meal (10%) mixture. A ration of hay (3·64 kg) and 4·55 kg of barley promoted gains which were not significantly different from those of animals fed additional nitrogen as soyabean meal or urea. This indicates that the foregoing ration supplied enough nitrogen to meet the animals' requirement.