Phosphorus fertilization of annual ryegrass and comparison of soil phosphorus extractants

It is unclear how much phosphorus ( P) fertilizer is needed to maximize annual ryegrass ( Lolium multiflorum Lam. cv. 'TAM90') forage yield and nutritive value in lower-rainfall climates. It is equally unclear which soil-P extractant is most effective when making P fertilization decisions. A 3-yr field study addressed these issues using small replicated plots during the winter months in north-central Texas. The addition of P increased yields every year of the trial, but optimum levels depended on rainfall distribution: 19 kg P ha(-1) yr(-1) was optimum two years, while 28 kg P ha(-1) yr(-1) increased yields 51% the year with good mid-season moisture. The addition of P fertilizer did not increase forage crude protein or affect fiber concentrations but did increase P yields. Percent relative yield of ryegrass was correlated with Mehlich III soil test P, soil pH, and P application rate. Conversely, the acidified NH4OAc-EDTA soil P extractant failed to relate to percent relative yield.