Ruminant Urine Increases Uptake but Decreases Relative Recovery of Nitrogen by Smooth Bromegrass

Core Ideas Forage mass and crude protein increase with N fertilizer rates up to 160 lb/acre in smooth bromegrass pastures. Smooth bromegrass responds even more so to urine from livestock grazing. Managers should encourage even grazing distribution across time to minimize nutrient losses from pastures. Understanding pasture responses to urine deposition remains limited. From 2011 to 2012, we investigated the effects of urine collected from ruminants and applied to N‐fertilized (0, 40, 80, 120, and 160 lb N/acre) plots (25 sq ft) of smooth bromegrass (Bromus inermis Leyss.) in eastern Nebraska. Urine was spread evenly across plots at both a volume (0.6 qt/sq ft) and N content (0.26 and 0.24 oz/qt) that simulated ruminant urine deposition in spring and resulted in an additional supply of 425 and 392 lb N/acre in 2011 and 2012, respectively. Control plots received the same volume of distilled water but no additional N. Response variables included forage mass, crude protein (CP) concentration, N uptake, apparent N recovery (ANR), and N use efficiency (NUE). In 2011, forage mass, CP, and N uptake increased linearly with N fertilizer rate and were 80, 30, and 135%, respectively, greater in urine‐treated than in distilled water–treated plots. In 2012, drought limited forage mass, and only urine impacted N responses. Apparent N recovery averaged 42% in urine‐treated and 63% in distilled water–treated plots in 2011 but did not differ between treatments in 2012, averaging 24%. Values for NUE did not differ among treatments and averaged 13.0 lb dry matter (DM)/lb N fertilizer applied in 2011 and 6.5 lb DM/lb N fertilizer applied in 2012. In years with favorable precipitation, producers can expect forage mass and CP to increase with fertilizer applications up to 160 lb N/acre. Optimum rates, though, would depend on production goals, nutritional needs of livestock, and fertilizer, land, and cattle prices.