PS10-12. Supplementation Level Strategies for Beef Heifers Grazing Warm-season Perennial Pastures.

Abstract In tropical regions, warm-season perennial grasses generally have greater forage accumulation and nutritive value during the summer, followed by a decline in early autumn. Consequently, adjusting supplementation levels may be an effective management strategy to optimize concentrate use in beef cattle grazing these forages. The objective of this study was to evaluate the effects of variable versus fixed supplementation levels on forage characteristics and animal performance of beef heifers grazing guineagrass (Megathyrsus maximus) pastures. The study was conducted at the Federal University of Northern Tocantins, Araguaina, TO, Brazil (Lat. 7o11’S Long. 48o12’W), from March to May 2024. Treatments were a fixed concentrate supplementation level (0.6% BW) or variable levels from 0.3% in March, 0.6% in April, and 0.7% in May. Treatments were distributed in a complete randomized design with four replicates. Twenty-four Nellore heifers (286 ± 4 kg BW) were assigned to 8 pastures (0.36 ha/pasture; 3 heifers/pasture). Pastures were subdivided into three paddocks and grazed with rotational and fixed stocking rates. Concentrate was 20% CP and 52% TDN and was fed daily. Pre-grazing herbage mass was estimated every 11 days and heifers were weighed every 33 days. Heifers were slaughtered and carcass characteristics collected at the end of the experimental period. There was no difference (P > 0.05) on pre-grazing forage height (77 cm) and herbage mass (9,300 kg DM/ha), and post-grazing herbage mass (6,500 kg/ha) between treatments; however, forage mass increased from 6,800 to 12,210 kg DM/ha from March to April and subsequently decreased to 9,100 kg DM/ha in May. Proportion of leaves in the canopy decreased (P < 0.01) from March (53%) to May (31%), which likely decreased forage nutritive value. Average daily gain was greater (P = 0.03) for variable (0.79 kg/d) than fixed levels (0.70 kg/d) of supplementation. Consequently, variable had greater gain per ha (P = 0.03) than fixed supplementation levels (575 vs. 523 kg BW/ha). Heifers receiving variable supplementation levels had greater (P = 0.01) carcass fat trim than fixed (5.8 vs. 4.7 kg); however, there was no difference (P > 0.05) in carcass dressing (53%), and longissimus muscle attributes: pH (5.6), backfat thickness (4.0 mm), and LA area (57.1 cm2), meat color (L* 37.5, a* 19.9, b* 8.1). Variable supplementation levels may be a viable strategy to optimize concentrate use for beef heifers grazing warm-season perennial grasses during the growing season.