Effect of defoliation severity on regrowth and nutritive value of perennial ryegrass dominant swards

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The height or mass to which swards are defoliated can potentially affect regrowth. A field study was undertaken to determine the response of perennial ryegrass (Lolium perenne L.) dominant swards to defoliation severity over repeated defoliations during a period of low water-soluble carbohydrate (WSC) accumulation (late spring to mid-autumn due to active leaf growth and relatively high respiration). Five defoliation severities (defoliation to a residual stubble height [RSH] of 20, 40, 60, 80, or 100 mm) were replicated five times in a Latin square design. During a 6-mo period, treatment plots were defoliated seven times using a rotary lawnmower. Analysis of data indicated a quadratic relationship between RSH and total herbage production, with total yields of 12,190, 13,440, 13,730, 13,320, and 11,300 kg DM ha–1 for swards defoliated to 20, 40, 60, 80, and 100 mm, respectively. Regression analyses identified 56 mm as the optimal RSH for herbage production, but there was little biological significance between 40 and 80 mm RSH. Total WSC content per tiller was reduced with increasing defoliation severity. The relationship between WSC content per tiller and subsequent herbage yield was quadratic, with peak herbage yield at 9.4 mg tiller–1 (60 mm RSH). The data indicate that between September (spring) and April (autumn), herbage production of temperate perennial ryegrass-dominant swards is maximized when defoliation severity results in post-grazing stubble heights of 40 to 80 mm.

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