Herbage production, botanical composition and survival of perennial ryegrass- and tall fescue-based swards in simple and diverse species mixtures in a dryland environment

Species diversity in grazed swards can improve sward DM production and stability when compared with traditional, simple binary perennial grass–clover combinations. In dairying regions faced with frequent seasonal soil moisture deficit, and environmental and economic incentives to lessen reliance on inputs (nitrogen fertiliser and irrigation) to overcome this, DM production from the traditional perennial ryegrass–clover swards can be reduced. Mitigating reduced DM production and feed deficits in the absence of inputs could potentially be achieved through increasing sward species diversity through the inclusion of additional forage species that are more productive at key times of the season when perennial ryegrass–clover swards commonly have reduced growth (i.e. summer). A 4-year, grazed small-plot experiment was conducted to evaluate sward DM production, species composition and sward survival of perennial ryegrass- and tall fescue-based swards with varying species compositions. The experimental design was a 2 × 6 factorial of two grasses, perennial ryegrass (PR) or tall fescue (TF), sown with white clover in six species combinations, as follows: no additional species (Control); with red clover (RedCl); lucerne (Luc); chicory and plantain (Herb); timothy and prairie grass (Grass); or a mix of all additional species (Mix). Mean annual DM production over the 4 years was greater (P < 0.05) from PR than from TF swards (13.4 vs. 12.9 t DM/ha/year), owing to superior DM production in 2 of the 4 years, largely during winter and autumn. The addition of other species to tall fescue-based swards did not improve annual or seasonal DM production. Increases in DM production (P < 0.05) were observed from PR-based swards sown with Grass, Herb or Mix in Year 3, when summer rainfall was 37% below the average (0.8–1.3 t DM/ha/year), but this effect was not seen in the following year when rainfall was even lower. Total sown-species content varied over the study period and was greater in PR-based than in TF-based swards (83 vs 70%, P < 0.01). Including additional species had varied effects on sown-species content; when differences were observed, it was greater sown-species content in the Herb and Mix than in the Control swards. However, by the end of the study, there was no observed improvement of sward survival with increased diversity. The findings of the present study suggested that there was no consistent advantage to increasing species diversity in PR- or TF-based swards on DM production or sward survival in dryland grazed dairy swards.