Project Details:

Annual ryegrass is becoming established as a major species for grazing stocker cattle in Southwest Texas, particularly along the southern edge of the Edwards Aquifer. The pressure to reduce ground water consumption in this region and the provisions of the Federal Agricultural Improvement and Reform Act of 1996 will likely increase the irrigated acreage used for stocker grazing. Yield response data for annual ryegrass grown under irrigation on the soils of this region are not available. Fertilizer recommendations from soil testing laboratories are based on results from dryland tests at other locations.

Surveys of protein and mineral composition or forage samples from ryegrass swards on Knippa clay and Uvalde clay loam soils indicate that bloat occurrence and death in cattle grazing these swards is usually accompanied by true protein values greater than 21%. Survey information also suggests that true protein in vegetative ryegrass grown on these soils will easily exceed 21% at fertility levels considerably less than those giving maximum growth response on silt loam soil in Louisiana (Robinson et al., 1987) or clay soil in the East Texas Coastal Plain (Westfall et al., 1971). By contract, one preliminary test showed that ryegrass grown on an East Texas sandy loam soil had equal crude protein but much lower true protein than ryegrass on these calcareous clay soils (H. Lippke, unpublished data).

The role of soil phosphorus levels in the soil-nitrogen/plant-nitrogen/plant- protein/bloat complex is unknown. Soil testing laboratories usually recommend relatively small amounts or no phosphorus application to these soils for growing ryegrass pastures. However, one year's yield data suggest that 80 lb P₂O₅/acre was not sufficient for high rates of ryegrass production (H. Lippke, unpublished data). Analyses of soil and forage samples associated with a preliminary response curve test have not been conducted.

This work is being undertaken to provide the basic response curve data for the effects of soil nitrogen and phosphorus on yield and forage composition of annual ryegrass on calcareous soils of Southwest Texas.