Early Season P Nutrition of Forage Grasses.

Early season P nutrition is known to be important for several annual species but less information exists for perennial forage grasses. Our objective was to determine the influence of P fertilizer on the primary growth of forage grasses.  Four P rates (0, 10, 20, and 40 kg P ha^-1) were applied at Lévis (CA), Normandin (CA), and Maaninka (FI) where timothy was the dominant grass species, while three P rates (0, 8, and 17 kg P ha^-1) in Changins (CH) and two P rates (0 and 50 kg P ha^-1) in Ercé (FR) were used with multi-species-based swards from long-term experiments. In the timothy-based experiments, increasing P fertilization did not significantly affect shoot biomass on most sampling dates of the spring regrowth in most years at Lévis, Maaninka, and Charlottetown but it increased increased shoot biomass on most sampling dates and in most years at Normandin. In the long-term multi-species experiments, increasing P fertilization significantly increased shoot biomass on most sampling dates at Ercé and Changins. A parallel curve analysis indicated that shoot growth rates were not affected by P rates at any of the site-years but the estimated shoot biomass on the first day of sampling, however, was increased by P fertilization at Normandin in 2012, 2013, and 2014, at Ercé in all four years, and at Changins in the two years. Early season growth from the initiation of growth to the first day of sampling was therefore affected by P fertilization at three of the six sites but shoot growth rates later during primary growth was not affected at all site-years. Our results confirm the importance of an adequate P supply during early grass growth.