Residual fertilizer effects on dry-matter yield and nutritive value of Mediterranean pastures

Five pastures in an Mediterranean environment in southern Italy were fertilized for 6 years (1993-1998) with nitrogen (N) fertilizer, phosphorus (P) fertilizer and two levels low (L) and high (H) of combined nitrogen and phosphorus fertilizer (N-P), and compared with a no-fertilizer control treatment, and the effects on soil variables, dry matter (DM) yield of herbage and floristic composition of the pastures measured. From 1998-2002, half of each plot was fertilized with the same treatments (continued treatment) while the other half received no fertilizer (discontinued treatment). In the year 2001-2002, the plots on the discontinued fertilizer treatments and the plots on the control treatment were used to evaluate the residual effects of the fertilizer treatments. The P and N-P fertilizer continued and discontinued treatments, in comparison to the control treatment, had a higher content of available P₂O₅ in the soil while under the N fertilizer treatment it decreased. The N and P fertilizer treatments and the residual effects of these treatments led to higher DM yields and proportions of Gramineae and Leguminosae, respectively, compared to the control treatment. Furthermore, differences in the nutritive value of the herbage and floristic composition were observed between the N-P fertilizer treatments and the control treatment. The residual effect of the N, P and N-P fertilizer treatments increased DM yield proportionately by 0.063, 0.385 and 0.404, respectively, and reduced the crude protein content of the herbage. The residual effects of the fertilizer treatments on milk forage units (MFU) were 261 for N, 1107 for P and 1003 MFU ha⁻¹ for N-P fertilizer treatments. Among fertilizers, the residual effect of the N-P fertilizer treatment promoted an increase in DM yield but produced little variation in floristic composition of the pasture. In general, the fertilizer treatments increased DM yield and nutritive value of herbage and reduced floristic composition in pastures on these Mediterranean sites.