Evaluation of Nitrogen-Delivery Methods for Stocker Cattle Grazing Annual Ryegrass
Type of DegreePhD Dissertation
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A 3-yr study was conducted to evaluate the efficacy of replacing N fertilizer with either interseeded annual legumes or supplementation with high-protein by-product feeds for stocker cattle grazing annual ryegrass. Each yr, 90 steers (initial BW, 225 ± 36 kg) were assigned to the following treatments, with or without monensin fed in a free-choice mineral supplement: 1) fertilization of annual ryegrass (Lolium multiflorum Lam.) with 112 kg N/ha in split application (NFERT), 2) fertilization of annual ryegrass with 56 kg N/ha at planting and interseeded crimson clover (Trifolium incarnatum L.; CC), 3) fertilization of annual ryegrass with 56 kg N/ha at planting and interseeded arrowleaf clover (Trifolium vesiculosum Savi; AC), 4) fertilization of annual ryegrass with 56 kg N/ha and supplementation of dried distillers grains plus solubles at 0.65% BW daily (DDGS), and 5) fertilization of annual ryegrass with 56 kg N/ha and supplementation with whole cottonseed at 0.65% BW daily (WCS). Grazing was initiated on December 14, 2015 (Yr 1), February 15, 2017 (Yr 2), and December 21, 2017 (Yr 3). Steers were weighed unshrunk every 28 d, and forage mass (FM) was measured concurrently using the destructive harvest/disk meter double-sampling method. Thirty 0.81-ha paddocks were stocked initially with three ‘tester’ steers, and stocking density was adjusted using ‘put-and-take’ steers based on changes in FM and steer BW to maintain a forage allowance (FA) of 1 kg DM/kg steer BW. Grazing was discontinued on May 11, 2016 in Yr 1, May 10, 2017 in Yr 2, and February 19, 2018 in Yr 3 following 140, 84, and 56 d of grazing, respectively. Data for all steers (i.e., tester and put-and-take steers) were used to determine stocking density, forage allowance, and grazing-d/ha. Total gain/ha was calculated for each pasture as the ADG of tester steers and grazing-d/ha of both tester and put-and-take steers (Beck et al., 2011). Dependent variables evaluated included ADG, total gain/ha, stocking density, and FA, and FM. Forage nutritive and chemical compositional data included the dependent variables IVTD and concentrations of CP and DIP. Data were analyzed using PROC MIXED of SAS 9.4 (SAS Inst. Inc., Cary, NC.) for a 5 × 2 factorial design consisting of 5 N-delivery methods and 2 levels (+/-) of monensin. Dependent variables for pasture botanical composition included pasture clover percentage and pasture clover DM mass. Clover mass was calculated as total forage DM mass multiplied by clover percentage per pasture. Data were analyzed using PROC MIXED of SAS 9.4 (SAS Inst. Inc., Cary, N.C.) for a 2 × 2 factorial design consisting of two clover species and two levels (+/-) of monensin. Main effects were N-delivery method, ionophore, year, and their 2-way interactions. Because there were no significant 3-way interactions detected for any of the dependent variables evaluated, their sum of squares and associated df were apportioned to the model error term (residual) for significance testing. The PDIFF option of LSMEANS was used to separate means when protected by F-test at α = 0.10. Forage mass (FM) was affected by yr (P < 0.0001) and N-delivery method (P = 0.004) with FM being greatest in Yr 2, intermediate in Yr 1, and least in Yr 3 as a result of differences in temperature and precipitation experienced among the 3-yr of the study. Forage mass was greatest for NFERT, DDGS, and WCS, intermediate for CC, and least for AC. Average daily gain (P = 0.001), total gain/ha (P < 0.0001), stocking density (P < 0.0001), and grazing-days (P < 0.0001) were greater for NFERT, DDGS, and WCS than CC and AC. Average daily gain (P < 0.0001), total gain/ha P < 0.0001, stocking density (P < 0.0001), and grazing-days (P < 0.0001) were affected by yr. Forage IVTD (P < 0.0001), CP (P < 0.0001), and DIP (P < 0.0001) were affected by yr, and DIP was also affected by N-delivery method (P = 0.0006) such that NFERT, AC, DDGS, and WCS were greater than CC. Clover presence (P = 0.001; P < 0.0001) and clover DM mass (P = 0.003; P < 0.0001) were affected by year and clover species, respectively. Monensin inclusion affected clover presence (P = 0.01) and clover mass (P = 0.02). Results are interpreted to mean that supplementation with a high-protein by-product feed for cattle grazing annual ryegrass maintained or improved cattle performance characteristics, stocking densities, and grazing-days compared with fertilized annual ryegrass or annual ryegrass interseeded with annual clovers, and that yr more so than N-delivery method affected forage nutritive value.
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