Irrigated Annual Ryegrass Responses to Nitrogen and Phosphorus on Calcareous Soil

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Abstract
Annual ryegrass (Lolium multiflorum Lam.) is increasingly being planted under irrigation as forage for stocker cattle in south central USA, but fertilizer recommendations for maximum production are not well defined. Labile N and P are generally deficient in this region's calcareous soils; K availability is usually considered adequate for plant growth. Growth responses of annual ryegrass to a factorial array of five levels of N and four levels of P were studied for 3 yr on a Knippa clay soil (fine, mixed, superactive, thermic Vertic Calciustolls). Responses to N and P were generally curvilinear, but they also had a strong linear component in Year 1. Increments of growth responses to levels of fertilizer above 269 kg applied N or 39 kg applied P ha−1 were relatively small. Response surface models projected maximum annual dry matter yields that averaged 9.23 t ha−1 and were associated with predicted requirements for applied N and P that averaged 488 and 61 kg ha−1, respectively. Economically optimal levels of applied N were predicted to range from 250 to 315 kg ha−1; the predicted economically optimal range for applied P was 31 to 41 kg ha−1. Nitrogen concentration in ryegrass forage increased as applied N increased. The data suggest that fertilizing vegetative ryegrass to maintain N in leaf tissue ≥32 g kg−1 provides economically optimal growth for both the ryegrass crop and the young cattle grazing it.

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