PSI-4 Stair-step supplementation effects on early-weaned beef calves receiving annual ryegrass.

Abstract The objective of this study was to evaluate the effects of increasing supplementation levels strategy on performance of early-weaned beef calves consuming annual ryegrass (Lolium multiflorium). Two experiments were conducted at the UF/IFAS Range Cattle Research and Education Center, Ona, FL from January to June 2024. In experiment 1, 32 Angus crossbred calves (78±14 kg BW and 112±14 d of age) were assigned to 8 annual ryegrass pastures (0.3 ha; 4 calves/pasture). Treatments were a fixed concentrate supplementation level (0.75% BW) during the experimental period or stair-step levels from 0.5% BW in February, 0.75% BW in March, and 1.0% BW in April. Treatments were distributed in a randomized complete block design with 4 replicates. Concentrate (19.8% CP, 72% TDN) was fed daily. Herbage mass and nutritive value were estimated every 14-d, and calves were weighed every 28-d. There was no significant differences (P ≥ 0.05) between treatments for herbage mass (mean = 2730 kg DM/ha), herbage allowance (1.9 kg DM/kg BW), CP (mean = 27%), in vitro digestible organic matter (IVDOM, mean = 75%), ADG (mean = 0.73 kg/d), glucose (mean = 103.7 mg/dL), BUN (mean = 20.4 mg/dL), and IGF-1 (mean = 42.3 ng/ml). In experiment 2, 24 calves (130±15 kg BW) from experiment 1 were transferred to a feedlot (2 steers/pen) and maintained in the same treatments. Forage and total DM intake, as well as in vivo DM digestibility, were measured. Annual ryegrass hay (13% CP, 73% IVDOM) was offered with 10% refusals allowed. The stair-step supplementation treatments were increased after 17-d, 10-d adaptation and 7-d collection periods. There was a treatment × cycle interaction for forage and total DM intake (P < 0.01). Forage DM intake did not differ between treatments in cycle 1 (mean = 1.77% of BW; P = 0.78) but was greater for the stair-step treatment in cycle 2 (1.81 vs. 1.75% BW; P = 0.03). In cycle 3, calves receiving fixed amount of supplement had greater forage intake (1.95 vs. 1.87% BW; P = 0.02). Total intake was greater for the fixed group than stair-step treatment (2.45 vs. 2.22% BW; P < 0.01) in cycle 1, while stair-step had greater total intake in cycles 2 and 3. Additionally, there was a treatment × cycle interaction for in vivo digestibility (P < 0.01). The fixed group had greater digestibility than the stair-step in cycle 1 (71.5 vs. 68.9%; P < 0.01), while stair-step had greater diet digestibility in cycle 3 (77.1 vs. 75.4%; P < 0.01). There was no difference in digestibility between treatments in cycle 2 (mean = 75.5%; P = 0.94). The stair-step supplementation strategy may not optimize forage intake and digestibility and increase performance of early-weaned calves receiving annual ryegrass.