Abstract
Abstract Optimizing mineral supplementation is critical to maximizing production in a forage-based beef production system. The effect of nitrogen fertilization on mineral accumulation in tall fescue (TF) is not well understood. We evaluated the differences in mineral composition in nitrogen fertilized (TF+N) and unfertilized (TF-N) tall fescue pastures. The experiment was conducted at the University of Missouri Forage Systems Research Center during 2017 and 2018. A randomized complete block design was arranged in novel-endophyte TF pastures. All pastures were mechanically harvested in May. Nitrogen was applied to TF+N in June and again in August (45 kg ha-1 and 90 kg ha-1, respectively). Treatments were rotationally grazed from July to November using yearling cattle. Randomly selected 10 m2 strips were harvested biweekly and 300 g subsamples were dried. Nitric acid digestion was performed followed by ICP-MS to determine mineral content. Data were normalized using rank transformation and analyzed by treatment and month. Analysis of variance was conducted using PROC GLIMMIX (SAS 9.4); slice command was used to determine interactions within month by treatment and within treatment by month. Calcium and magnesium concentration decreased throughout the season with the lowest concentrations observed in both treatments during November (P = 0.0043 and P = 0.0007, respectively). Greater potassium concentrations were observed in June compared to November in both treatments (P < 0.0334). We observed a within month by treatment interaction for sodium. Nitrogen fertilization resulted in greater sodium concentration in all months from August to November compared to TF-N (P < 0.0074). Greater sodium concentrations resulted in decreased K:Na ratios during September, October, and November in TF+N (P = 0.0013, P = 0.0036, and P = 0.0024, respectively). Our results suggest nitrogen fertilization reduced K:Na ratio in TF pastures, which could negatively affect magnesium absorption in recticulo-rumen.
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