Effects of grazing intensity and advancing season on chemical composition and in vitro organic matter disappearance in steers grazing mixed-grass prairie

Abstract Text: A study was conducted to evaluate the influence of advancing season and grazing intensity on dietary chemical composition and in vitro organic matter disappearance (IVOMD) in beef steers grazing mixed-grass prairie in the Missouri Coteau of south central North Dakota. Five sampling periods were conducted from mid-May to early September 2015. Twelve ruminal cannulated crossbred steers were used to collect diets while 188 crossbred steers were used to maintain specific grazing intensities on 12 pastures. Treatments were light (LT), moderate (MOD), heavy (HVY), and extreme (EXT) grazing intensities. Each treatment was assigned to 3 pastures. Grazing treatment × sampling period interactions were not present (P ≥ 0.29) for all variables measured except IVOMD (P ˂ 0.01). There were no main effects of grazing treatment for NDF, ADF, total N, soluble N (SN), insoluble N (IN), and ADIN. Responses to grazing season were evaluated with linear, quadratic, and cubic contrasts. Neutral detergent fiber increased linearly (P < 0.01) and cubically (P = 0.01), while ADF tended (P = 0.17) to increase linearly with advancing season. Dietary N decreased linearly (P < 0.01), quadratically (P = 0.01), and cubically (P = 0.01). Soluble N and IN expressed a linear (P < 0.001) and quadratic (P = 0.03) decrease across advancing season, while IN also showed a cubic response (P < 0.001). Acid detergent insoluble N did not change as season advanced (P > 0.14). In vitro OM digestibility decreased from May to September (P < 0.01) in all sampling periods, but did not show any trends across treatments (P = 0.82). However, IVOMD did show a treatment × period interaction (P < 0.01). In summary, these data indicate increases (P < 0.001) in dietary NDF and decreases (P < 0.001) in N, SN, IN, and IVOMD with advancing season. These data suggest seasonal factors are a more important driver of grazed masticate forage nutrient composition than the grazing intensities evaluated in this study. 

Keywords: dietary nutrient composition, grazing intensity, season