Nitrate and vitamin A interrelationships in sheep.

Abstract

Experiments were conducted to determine the effect of 2.5% sodium nitrate on performance, plasma vitamin A levels and hepatic vitamin A stores of sheep during periods of modest (4,000 I.U. vitamin A or 8 mg. carotene per Iamb daily) vitamin A supplementation, vitamin A depletion and vitamin A repletion (10,000 I.U. vitamin A or 20 mg. carotene per lamb daily). Nitrate significantly (P<,.05) reduced weight gains in lambs being depleted or receiving modest supplements of vitamin A or carotene, but it did not have the same effect during a repletion period when the lambs were considerably larger and making smaller weight gains. Carotene fed at a rate of 20 mg. per head daily from dehydrated alfalfa meal was only 56 to 57% as effective as 10,000 I.U. of vitamin A in the repletion of hepatic vitamin A stores of depleted lambs. This relative effectiveness of carotene vs. vitamin A was higher than was associated with lower levels of supplementation in non-depleted lambs. The feeding of 2.5% sodium nitrate did not appear to influence the relative effectiveness of carotene compared with vitamin A. Significantly (P<.05) lower plasma vitamin A and lower (P<.10) liver vitamin A values accompanied the feeding of nitrate to lambs supplemented with either carotene or vitamin A following vitamin A depletion. These effects of nitrate during vitamin A repletion were more pronounced than those observed during vitamin A depletion. It is concluded that dietary nitrate exerts its greatest influence on the vitamin A status of ruminants through a reduction in the amount of dietary vitamin A reaching hepatic stores rather than through an accelerated depletion of existing stores.