Effect of nitrogen source, rate, and timing on forage yield and quality in bermudagrass pasture systems

The Oklahoma beef industry faces a significant decline in cattle population due to drought-driven forage shortages and increasing winter-feeding costs which account for over 50% of total costs in cow calf operations. While dormant bermudagrass provides limited winter forage, stockpiling and sod-seeding present potential strategies to extend the grazing season. However, the effectiveness of these approaches depends on effective nitrogen fertilization. This study evaluated the impact of nitrogen (N) source, rate, and application timing on forage yield and nutritional quality focusing on stockpiling and sod-seeded bermudagrass pasture systems. The research utilized three N sources (ammonium nitrate, urea, and urea + NBPT), four N rates (0, 44, 89, and 134 kg N ha⁻¹), and two application timings (single and split). Sixteen treatments in a split block design with three replications was established on existing bermudagrass pasture at three locations: Lindley Research and Demonstration Farm in Valliant, Cimarron Valley Research Station in Perkins, and the South-Central Research Station in Chickasha, Oklahoma from August 2024 to December 2024. Split-block and randomized complete block models were applied to evaluate the impact of the N treatments on the response variables using R software. At Valliant, N rate significantly affected forage yield, with 80 kg N ha⁻¹ producing the optimal yield (3124 kg DM ha⁻¹), while more N beyond the optimal rate decreased yield by 24.4%. At Perkins, N source and timing interaction was the main driver of yield than rate, with urea + NBPT significantly outperforming untreated urea by 49% when applied as a single application while split application improved urea performance at higher rates. Sod-seeding produced forage with significantly higher crude protein (9.1% vs. 7.2%) and TDN (57.92% vs. 53.98%) compared to stockpiling. Nitrogen use efficiency decreased significantly with increasing N rates, with the highest efficiency (82.91% at Valliant and 63.73% at Perkins) observed at 44 kg N ha⁻¹. These results highlight the importance of site-specific N management and demonstrate that sod-seeding cool-season annuals into bermudagrass can significantly improve forage nutritive quality for winter grazing while meeting nutritional requirements for dry cows and approaching adequacy for replacement heifers.