374 Effects of breed and trace mineral source on performance of beef heifers during nutrient restriction and grazing forage at early vegetative stage.

Abstract This study evaluated how breed and mineral source affect beef heifer performance and physiology during nutrient restriction and grazing forage at early vegetative stage. On day -7, ½ Angus × ½ Nelore (ANE) and Nelore (NE) heifers (n = 12 heifers/breed; 264 ± 35 kg; age = 15 ± 1 months) were transferred to individual drylot pens and offered free choice access to Tifton 85 (Cynodon sp.) hay for 7 days. On day 0, within each breed, heifers were randomly allocated (2 ´ 2 factorial design) to receive protein supplementation (0.20% of body weight) added with sulfate (SUL) or hydroxychloride (HYD) sources of Cu, Mn, and Zn from day 0 to 49. From day 0 to 34, hay intake was restricted to 50%, followed by grazing Tifton 85 forage at an early vegetative stage until day 49. Non-binary and binary data were analyzed using the MIXED and GLIMMIX procedures of ASAS. Effects of breed × mineral source × day and breed × mineral source were not detected (P ≥ 0.11) for any variable. Nelore heifers had greater (P ≤ 0.02) average daily gain (ADG) from day 0 to 49 compared to ANE heifers, with no breed effects on diarrhea incidence, fecal pH, and total days of diarrhea symptoms (P ≥ 0.19). Nelore heifers had greater plasma insulin-like growth factor 1 concentrations (P ≤ 0.05) but less plasma cortisol and haptoglobin concentrations (P ≤ 0.05) compared to ANE heifers. Serum concentrations of urea N were greater (P ≤ 0.05) for NE compared to ANE heifers on days 0 and 42 and were less (P ≤ 0.05) for NE compared to ANE on days 38, 45 and 49. Regardless of breed, heifers supplemented with HYD had greater ADG (P ≤ 0.05) during nutrient restriction, lower diarrhea incidence from days 36 to 49 and percentage of heifers exhibiting ≥ 2 days of diarrhea from days 36 to 49, and reduced serum non-esterified fatty acids on day 35, and serum cortisol and urea N concentrations on day 38 compared to SUL heifers. Overall, breed influenced physiological markers and growth during nutrient restriction but did not affect diarrhea incidence during grazing forage. Regardless of breed, HYD supplementation improved growth and reduced diarrhea incidence compared to SUL supplementation.