Resumen
- Forty-eight crossbred beef calves (initial BW = 772 lb) were used to determine the effects of live weight gain and conjugated linoleic acid (CLA) on immunity and growth performance during a 56-d study. Calves were blocked by weight, stratified by gen- der, and assigned to 16 pens. Pens within each block were assigned randomly to 1 of 4 diets arranged as a 2 x 2 factorial. Main effects were rate of gain (diets formulated for calves to gain 1.5 (L) or 3.0 (H) lb/d), and fatty acid source (4% Ca salts of palm oil (PO) or 4% Ca salts of CLA). Feeding CLA tended to increase (P = 0.07) ADG from d 1 to 56, and decrease (P = 0.06) F/G from d 29 to 56 in cattle fed L diets, and tended to decrease the percentage of monocytes (P = 0.09) and eosinophils (P = 0.06) in H diets. Total white blood cell (P = 0.02) and lymphocyte concentrations (P = 0.05) were greater in L vs. H diets. Feeding CLA decreased the proliferation of peripheral blood mononuclear cells (PBMC) stimulated with phytohemagglutinin (PHA; P = 0.04) or concanavalin A (P = 0.03) in H but not in L diets. Responses of PBMC to PHA were lower (P = 0.003) in L than in H diets. Monocyte/macrophage phagocytosis was not affected by CLA or rate of gain (P > 0.22). In conclusion, CLA may increase growth performance and modulate immunity in growing cattle but responses depend on time and rate of growth