Sulphur metabolism and excretion studies in ruminants. XIII.* Intake and utilizaton of wheat straw by sheep and cattle

Abstract

The capacity of cattle and sheep to utilize wheat straw was compared, six 15-month-old Hereford steers and six 20-month-old Merino wethers being used.

The addition of urea (2.0% of dry matter) to wheat straw (c. 0.3% nitrogen, 0.09% sulphur) significantly increased the intake of energy, the efficiency of digestion and the liveweight gains of both the sheep and the cattle. The addition of sodium sulphate (0.4% of the dry matter) to the ration containing urea significantly increased these responses in the sheep but not in the cattle. Cattle and sheep had a similar capacity to digest the basal straw (22 v. 23 kcal/kg W^0.92 per day), but cattle were more efficient than sheep when urea alone was included in the ration (68 v. 42 kcal/kg W^0.92 per day day). The further addition of sulphate decreased this disparity (75 v. 64 kcal/kg W^0.92 per day) so that the sheep were then also able to derive their maintenance requirement of energy (an estimated 49 kcal/kg W^0.92 per day) from the straw.

Addition of urea to wheat straw significantly increased the nitrogen balance in cattle but not in sheep. The further addition of sulphate significantly increased this balance in both the sheep and the cattle. Nitrogen balance on the basal straw, straw plus urea, or straw plus urea plus sulphate was –97, –62 and 17 mg nitrogen/kg W^0.92 per day respectively for sheep compared with –79, –24 and 32 mg nitrogen/kg W^0.92 per day for cattle.

Supplemental sodium sulphate (0.4% of the dry matter) without urea slightly decreased the nitrogen balance and tended to depress intake and digestion of feed in both sheep and cattle.

Cattle require less dietary sulphur and can cope with a wider nitrogen/sulphur ratio in the feed than sheep, apparently because sulphur is recycled more effectively in cattle. Responses to urea supplementation under grazing conditions may therefore be greater with cattle.

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*Part XII, Aust. J. Biol. Sci., 26: 1429 (1973).p>