Effects of height and frequency of defoliation on the productivity of irrigated oats (Avena strigosa cv. Saia) and perennial ryegrass (Lolium perenne cv. Kangaroo Valley), grown alone or with barrel medic (Medicago truncatula cv. Jemalong)

Abstract

The effects of varying the height (5- 10 cm) and frequency (2-6 weeks) of defoliation on the dry matter yield, numbers of plants and tillers, and forage quality of Avena strigosa cv. Saia (oats) and Lolium perenne cv. Kangaroo Valley (perennial ryegrass), grown both in pure swards and in combination with Medicago truncatula cv. Jemalong (barrel medic) were evaluated in irrigated pastures at Gatton in southeastern Queensland in 3 experiments over 2 years. In pure swards, defoliation every 6 weeks at the lowest cutting height (5 cm) gave the highest yields of both oats (10 t DM/ha) and ryegrass (12.6 t DM/ha). There were fewer live tillers per m2 of both oats and ryegrass cut at 6-week intervals compared with more frequent defoliations. However, tiller numbers of both were unaffected by cutting height. Lengthening the regrowth period from 2 to 6 weeks reduced nitrogen concentration by 23% (from 4, 75%) for ryegrass and by 16% (from 4.22%) for oats but increased cell wall content of oats from 38 to 45% and ryegrass from 36 to 41%. In vitro digestibility of oats fell from 82.4 to 79.9%, but the digestibility of ryegrass was unaffected. Cutting height had no effect on these measurements for either forage. In mixtures, ryegrass DM yield was highest (3.6 t/ha) when cut every 3 weeks and at the lowest defoliation height (5 cm) whereas cutting at 6-week intervals gave the highest yields of oats (5.2 t/ha). Legume yield was usually highest with defoliations every 3 or 4 weeks and at the lowest cutting height (5 cm), and was higher when grown with ryegrass (4.4 t/ha) than with oats (4.0 t/ha).