PS13-11. Effect of Forb Inclusion on Digestion and Metabolism of Warm-Season Forage Diets in Beef Cattle.

Abstract Warm-season forbs have gained attention in grazing systems for their high productivity, favorable nutritive value, and potential to reduce enteric methane (CH4) emissions in beef cattle. However, variable palatability and limited voluntary intake often prevent achieving targeted inclusion rates. Many of these species contain naturally-occurring plant secondary metabolites (PSM), which may alter rumen fermentation and CH4 production. The objective of this study was to investigate how forb inclusion in grass-based diets affect beef cattle. A 4 × 4 Latin square in vivo metabolism experiment was conducted, with columns corresponding to experimental periods (n = 4) and rows to ruminally-fistulated steers (n = 4). Dietary treatments consisted of a 70:30 ratio of bahiagrass (Paspalum notatum Flueggé.) with 30% sunn hemp (Crotalaria juncea L.; SUN), lablab (Lablab purpureus [L.] Sweet; LAB), or soybean (Glycine max [L.] Merr.; SOY). A grass-only treatment served as the control (CON). Each experimental period consisted of a 14-d adaptation to diet and facilities followed by 5 d of total fecal and urine collections and 3 d of ruminal collections. Multivariate analyses were conducted using PROC VARCLUS, PROC STEPDISC, and PROC CANDISC, while univariate analyses were performed using PROC GLIMMIX. The SOY treatment was consumed at 12.1% of DMI, which was greater (P < 0.05) than other treatments; however, overall forb intake remained below the targeted inclusion ratio of 30%. There was no effect of treatment (P ≥ 0.12) on the digestibility of any nutritive value parameter except ADF (P = 0.04). Apparently absorbed N did not differ among diets (P ≥ 0.39), similar to GE, DE and ME (P ≥ 0.06) which were also not affected by treatment. There was no effect of treatment on total VFA production (P = 0.11). However, proportion of isobutyrate and isovalerate were less (P < 0.05) from SOY than other treatments. There was no effect of treatment on potential activity of CH4 production (P = 0.28). While PSM demonstrate potential to mitigate enteric CH4 productions, their effectiveness is highly limited by palatability and voluntary intake of the targeted forbs. Future research should transition to large-scale grazing trials to evaluate the real-world efficacy of these forbs on overall animal performance.