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Nitrogen Management for Irrigated Annual Ryegrass in Southwestern United States¹

Annual ryegrass (Lolium multiflorum Lamb.) is becoming an important winter and spring pasture crop in the irrigated southwestern USA. A field study was conducted to determine the optimum N fertilization rates for producing maximum dry matter yields within tolerable NO^–₃-N concentrations for livestock and to develop a diagnostic test using plant and/or soil NO^–₃-N concentrations for recommending rates and frequency of N applications to ryegrass pastures. The cv. ‘Ucivex’ was seeded in an Imperial silty clay [fine, montmorillonitic (calcareous), hyperthermic Vertic Torrifluvents] in October, first harvested in December, and cut about monthly thereafter. In seven treatments, NH₄NO₃ was applied at seasonal rates of 0, 224, 448, 676, 896, and 1,120 kg N/ha, in five equal applications, or 90 kg N/ ha preplant plus 112 kg N/ha after each harvest.

Seasonal dry matter yield (Y) was related to N application rate (X) by the equation Y = 2.37 + 0.0272X + 0.0000118X² (r = 0.999). Hay nitrate concentration increased with N application rate and was excessive, for animal tolerance, with single N applications above 112 kg/ha for the second test. Protein, ash, and fiber content of the hay increased, N-free extract and total digestible nutrients decreased, and fat was unaffected by increase in N application rate. Soil nitrate concentrations of 10 to 20 ppm in the upper 60 cm ensured near maximal yields at next cutting without excessive hay nitrate concentrations. A hay content of 2,000 to 3,000 ppm NO^–₃-N was not adequate for satisfactory yield at the next cutting without additional N application. Multiple N applications at 112 kg/ha at about monthly intervals were recommended.

Key Words: Lolium multiflorum Lamb. • Forage and dry matter yield • Hay nitrate concentration • Soil nitrate concentration • Protein • Fiber • Ash • Fat • Nitrogen-free extract • Total digestible nutrients

² Plant physiologist, USDA, ARS, and Imperial County Farm Advisor, El Centre, CA.

Received for publication March 23, 1981.