Nitrogen fertilizer application and developmental stage affect silage quality of timothy (Phleum pratense L.)

Abstract

AbstractCrop composition at harvest affects the ensiling process and the resulting silage quality. The objectives of this study were to determine: (i) the effect of annual N‐fertilizer application (0, 60, 120 and 180 kg N ha−1) and developmental stage (stem elongation, early heading, late heading and early flowering) on the ensiling properties and silage quality of the spring regrowth of timothy (Phleum pratense L.) at two sites for 1 or 2 years, and (ii) the relationship between ensiling properties of the forage and the quality of the resulting silage. Laboratory silos with wilted forage at approximately 350 g dry matter (DM) kg−1 of fresh matter were prepared at each harvest and opened 150 d later for silage analysis. Higher rates of N‐fertilizer application decreased the concentration of water‐soluble carbohydrates (WSC), increased the buffering capacity (BC) and nitrate concentration, and decreased the ratio of WSC:BC, primarily in the early stages of development. The ensiling properties of timothy were, therefore, less favourable when high rates of N fertilizer were applied. Silage pH generally increased with increasing rates of N‐fertilizer application; this increase was particularly evident at the first three developmental stages at one site in 1 year. Non‐protein N (NPN) and soluble N concentrations of the silages increased with increased rates of N‐fertilizer application at the first three developmental stages but decreased at early flowering. Ammonia‐N concentration in the silages increased by 0·85, 0·56 and 0·67 when rates of N‐fertilizer application were 60, 120 and 180 kg ha−1, respectively, compared with that when no N fertilizer was applied. Significant correlations between the composition of the forage ensiled and silage quality variables were found at sites in individual years but, when all data were combined, WSC concentration and BC, and their ratio in the forages, were not correlated with pH, and soluble‐N and ammonia‐N concentrations of the silages, and were weakly correlated with NPN and free amino acid‐N concentrations of the silages. Silage quality was reduced by increased N‐fertilizer application, primarily at the early developmental stages, and this can be attributed to a reduction in WSC concentration and an increase in BC of the forage. Water‐soluble carbohydrate concentration, BC, and their ratio, however, were poor predictors of silage quality.