ANTIBODY-RESPONSES OF GROWING BEEF-CATTLE FED SILAGE DIETS WITH AND WITHOUT SELENIUM SUPPLEMENTATION

The relationships between blood Se levels and glutathione peroxidase ( GSH-Px) activities and the ability of cattle to produce antibodies in response to antigen challenges with sheep red blood cells (SRBC) and o valbumin (OA) were examined in a two-phase experiment with yearling be ef cattle. There were no treatment effects on weight gains, feed intak e or efficiency of feed conversion in either phase. In phase 1, cattle fed the unsupplemented control diet had lower (P less-than-or-equal-t o 0.05) blood Se levels and GSH-Px activities by week 8 than cattle fe d Se-fertilized forage, but neither was different (P > 0.05) from thos e of cattle fed Se-enriched yeast or inorganic Se. Antibody titers res ulting from injection of SRBC at week 4 of the experiment peaked about 3 wk after injection, then declined. Differences due to Se treatment were small. The same animals were used in phase 2, where half of the a nimals on each of die four treatments of phase 1 were fed a supplement with Se-enriched yeast and the other half were fed an unsupplemented control. Differences in blood Se levels and GSH-Px activities due to p hase-1 treatments persisted throughout phase 2. Differences due to pha se-2 treatments were apparent after 9 wk. The cattle were challenged w ith OA at week 12 and at week 17 with both OA and SRBC. Differences in antibody titer due to Se treatment were small but tended to be higher (P > 0.05) in the Se-supplemented cattle. These results support the s uggestion that blood Se levels over 100 mug L-1 are needed to maintain optimum immunocompetence.