Starch or Highly Digestible Fiber As Energy Supplement for Replacement Heifers Grazing Annual Ryegrass

Animal response to concentrate supplementation on grazing systems depends on pasture and supplement characteristics. The objective was to evaluate the effect of level of ground corn and soybean hulls on performance of replacement heifer grazing annual ryegrass. In three consecutive grazing seasons, 90 crossbred beef heifers (269±8.1 kg) were blocked by weight and genotype and assigned to 15 groups (n=6) that were continuously stocked on 1.33 ha pastures of annual ryegrass. Groups were then allotted to five treatments (3 replicates): 1) Control (CON), no supplement; 2) 0.5% BW ground corn (C05); 3) 0.5% BW soybean hulls (SBH05); 4) 1% BW ground corn (C1); and 5) 1% BW soybean hulls (SBH1). Forage mass, forage growth rate, forage nutritive value, and BW were determined on d 0 and every 15 d thereafter while amount of supplement fed was adjusted accordingly. Annual ryegrass (var. ‘Nelson’) was no-tilled at a seeding rate of 45 kg/ha and fertilized with 120 units of N/ha (urea) split in three applications. Grazing period started in January of each year and lasted for 75 d. Data were analyzed using Proc GLM for treatment, yr, and their interaction. There were no interactions on any variable so main factors are discussed. No treatment effect (P > 0.05) was detected for ADG (1.19, 1.39, 1.29, 1.36, and 1.30, for CON, C05, SBH05, C1, and SBH1, respectively) and nutritive value parameters, however forage mass (1023, 1401, 1359, 1991, and 2020 kg DM/ha, for CON, C05, SBH05, C1, and SBH1, respectively) and forage DMI (6.4, 5.7, 5.5, 4.6, and 4.2 kg DM/ha, for CON, C05, SBH05, C1, and SBH1, respectively) were affected (P= 0.002 and P=0.03, respectively). A yr effect (P= 0.004) was observed for ADG (1.27, 1.49, and 1.16 kg, for yr 1, 2, and 3, respectively), beef produced per ha, and forage mass (P= 0.04; 1621, 1931, and 1081 kg DM/ha for yr 1, 2, and 3, respectively). The amount of supplement fed in C1 and SBH1 (227 vs. 233 kg per animal, respectively) was different (P= 0.02). The efficiency of supplementation (kg supplement per kg gain over CON) was greater (P= 0.002) for C05 (7.8), intermediate for C1 (17.8) and SBH05 (15.5), and lowest for SBH1 (28.2). Both supplements reduced forage DMI with greater substitution effect at the 1% BW level. Larger yearly variation in forage production influenced animal performance more than level and type of supplement used.