PSI-17 Impact of within-field variability in soybean yield on nutrient composition and inclusion levels of meal in beef cattle diets.

Abstract Soybean meal (SBM) is a high-quality protein source in ruminant diets due to its high crude protein (CP) and well-balanced amino acid content. However, the yield and nutritional composition of seeds can vary depending on edaphic, biological, anthropogenic, topographic, and meteorological factors. These variations may impact the value of SBM as a feed ingredient, influencing dietary composition, animal performance, and profitability. This study evaluated how within-field soybean yield variability affects the nutrient composition of seeds and dietary inclusion rates of SBM in a model beef cattle diet. The research was conducted in a non-irrigated soybean field of Southern Illinois University (SIU), located in Elkville, IL. Soybean seed samples (n=27) were collected from previously identified high, medium, and low-yielding sites (n = 9 each) within the field. The nutrient composition, including CP and amino acid profiles, of the samples was analyzed. The value of SBM was calculated based on the composition of soybeans (CP, concentrations of lysine, methionine, tryptophan, and isoleucine) as described by Mourtzinis et al. (2018). The seed composition is converted to a “virtual meal” composition, assuming 12% moisture and 74% meal yield. A model diet for growing beef cattle (cross-bred yearling beef steers weighing 380 kg, gaining 1.5 kg/day) was formulated using the Beef Cattle Nutrient Requirements Model (BCNRM, 2016), consisting of 10% grass hay, 40% corn silage, 45% corn grain+SBM, and 5% vitamin-mineral supplement, with SBM inclusion adjusted based on the nutrient composition of individual SBM samples to have a positive balance for amino acids including arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine in the total mixed ration (TMR). Data were analyzed using the mixed model procedure of SAS 9.4, with Pearson correlation coefficients computed using the Proc Corr procedure to assess relationships between SBM value and nutrient concentrations. Results indicated that the SBM value ranged from $300.1 to $330.8 per tonne, with lower values observed in soybeans from low-yielding sites compared to medium-yielding sites ($307.5 vs. $317.54; P < 0.05), while high-yielding sites were intermediate ($313.5). The SBM value was positively correlated (P < 0.05) with the concentrations of CP and amino acid concentrations, which are critical for protein quality in livestock diets. SBM inclusion levels in the formulated diets varied from 192.8 to 232 kg/tonne of TMR, depending on the CP and amino acid concentrations. These findings emphasize that within-field yield variability significantly influences the quality of soybean seeds and resultant meals, impacting the inclusion rates of SBM in beef cattle diets, affecting beef cattle performance and farm profitability. By quantifying differences in CP and key amino acids, this research aims to refine the understanding of how spatial variability in soybean yields impacts beef cattle nutrition for economic livestock production.