The effect of urea on the utilization of ground, pelleted roughage by penned sheep: III. Mineral supplements

Abstract

SummarySeven, groups of adult Merino wethers were fed for 16 weeks on a diet of pelleted oat straw and urea, either alone or supplemented with trace minerals and various combinations of the major minerals. In some treatments, the same diet was fed throughout; in others, diets were changed after 8 weeks.Sulphur (S) was by far the most important mineral supplement; sheep fed diets supplemented with all minerals except S performed as poorly as those on the basal diet. Of the other minerals, only calcium (Ca) and magnesium (Mg), which were fed in combination, had a small, additional beneficial effect. Supplementary phosphorus (P) had no effect whether or not S was included.Intakes of sheep on S-supplemented diets increased over the first 6 weeks to reach stable levels of 1200–1400 g D.M./day compared with approximately 800 g/day for controls, while O.M. digestibilities stabilized at about 40 and 33% for S-supplemented and unsupplemented diets respectively. Live-weight change and wool growth were closely related to intake. Mean live-weight losses during the first and second 8 weeks for sheep without supplementary S were 9·5 and 4·8 kg respectively, and were about 50% of this for supplemented sheep. Sheep which were changed from diets without supplementary S to S-supplemented diets after 8 weeks showed slight increases in live weight from then on. Wool growth was about 115 g clean wool/sheep on all treatments during the first month, after which it remained steady at about 105 g/month on S-supplemented diets and fell to 60 g/month on diets without supplementary S.Sheep without supplementary S were in negative nitrogen (N) and S balance throughout. With the S-supplemented diets, urinary excretion of N and S was high, and few positive balances were recorded until the final 4 weeks of the experiment. Serum urea levels were high (60–130 mg/100 ml) throughout, and showed no effect of diet or period. Phosphorus balances were uniformly negative in sheep without supplementary P, and generally positive in P-supplemented sheep. Serum inorganic P levels were apparently affected by P intake and by live-weight loss, but mean values never fell below 4·5 mg/100 ml.Sheep without supplementary Ca were in negative Ca balance throughout the experiment; however, Ca supplementation generally resulted in high faecal Ca losses and few positive balances were recorded. In some sheep, serum Ca fell to moderately low levels (7·5 mg/100 ml), but this appeared to be associated more with high serum inorganic P levels than low Ca intake or retention. All sheep were practically in Mg, Na and K balance throughout, and supplementation had little effect on the retention of these elements.All sheep were in very poor condition at the end of the experiment, even when their D.M. intakes had been high, and wool growth rates were mediocre throughout. It appears that urea was poorly utilized in this experiment, and that satisfactory sheep production on diets of ground straw supplemented only with urea and minerals is unlikely.