Effect of defoliation height and redefoliation interval on regrowth and survival of perennial ryegrass (Lolium perenne) in subtropical dairy pastures

Abstract

A cut plot study was undertaken on the subtropical north coast of New South Wales, Australia, to determine the effect of defoliation height and redefoliation interval on dry matter yield and persistence of perennial ryegrass (Lolium perenne L.) pastures. The pasture was established on 7 April 1998 and plots were irrigated to replace evapotranspiration loss.

The study was a completely randomised block design with plots of 2 by 1 m and treatments replicated 3 times. In winter (commencing 13 July) plots were defoliated to 20, 50 or 120 mm stubble height and either not redefoliated or redefoliated at 3, 6 or 3 and 6 days after initial defoliation. In spring (commencing 28 October) plots were redefoliated as for winter but only to 50 mm stubble height. After imposition of the redefoliation treatments, the plots were allowed to regrow until the non-redefoliated treatments had regrown 3 new leaves per tiller (subsequently referred to as a regrowth cycle) and then again defoliated (regrowth cycle 1). Plots cut in winter were then halved with one half (A plots) continuing to be subject to the redefoliation treatment for 4 more regrowth cycles until regrowth cycle 1 in spring was completed on 24 November, while the other half (B plots) were a carryover comparison of redefoliation treatment in regrowth cycle 1. Both A and B plots continued to be subjected to the same defoliation height treatments as imposed in regrowth cycle 1. From 24 November to 30 March 1999, plots were defoliated at 50 mm height each time 3 new leaves per tiller had regrown.

Plots defoliated to 20 or 50 mm height during regrowth cycle 1 in winter yielded 21% more dry matter than plots cut to 120 mm height while redefoliation at 6 or 3 and at 6 days produced 14% less dry matter than plots not redefoliated or redefoliated at 3 days. Continued redefoliation at 6 days (comparison within A plots) reduced dry matter yield by 63% compared with no redefoliation or redefoliation at 3 days, but only in plots defoliated to 20 or 50 mm height.

Plant density in the autumn (March 1999) of the year after establishment was positively related to defoliation height over regrowth cycles 1–5 of the previous year (35, 55 or 77 plants/m² for plots defoliated at 20, 50 or 120 mm, respectively). Plant density of plots not redefoliated or redefoliated at 3 days over regrowth cycles 1–5 was 63% higher (70 plants/m²) than for the other treatment combination (43 plants/m²) at P = 0.07 level of significance.

Plants cut to 20 or 50 mm stubble height at the commencement of regrowth cycle 1 in winter had a stubble water-soluble carbohydrate content of 5.2%, decreasing to 2.3% at day 6 post-defoliation. The water-soluble carbohydrate content of plants cut to 120 mm were initially higher at 8% and fell to only 6.4% by day 6.

The redefoliation treatments imposed in this study were designed to simulate the regrazing of regrowth shoots in an extended grazing bout at various defoliation heights. The results confirm the negative effects of redefoliation, at 6 days in the winter to spring period, on both dry matter yield and plant survival over the subsequent summer in the subtropics. In contrast to winter, redefoliation in late spring had no effect on dry matter yield or plant density. The results also indicate a compromise between the benefits of more lax grazing for persistence and harder grazing for pasture utilisation.