Spray-topping annual grass pasture with glyphosate to delay loss of feeding value during summer. I. Effects on pasture yield and nutritive value

Annual grass pasture (mainly annual ryegrass) was treated at seed head emer gence (12 October 1990) with the herbicide glyphosate to delay loss of dige stibility during late spring and summer. The lowest rate of glyphosate appl ication needed to retain a high digestibility in the dry matter (DM) of the senescent grass sward was 180 g a.i./ha. The in vitro DM digestibility of control (unsprayed) pasture declined rapidly from relatively high levels ne ar anthesis (28 October) (77-86%, leaves and seed head; 66%, stems) until a bout 38 days after anthesis (51-71%, leaves and seed head; 24%, stem). Ther eafter, the digestibility of the plant parts did not change appreciably ove r at least 4 months. Treatment with glyphosate significantly slowed the los s in digestibility of stem, leaf sheaths, and seed head but did not affect the decline in digestibility of leaf blades. For example, the digestibility of stem in the sprayed pasture was 26 percentage units higher than that in the control pasture 38 days after anthesis. If the time at which digestibi lity of stems reached 50% is taken as an indication, the effective delay in loss of digestibility was of the order of 5 weeks. However, improved diges tibility in the treated pasture was achieved at the expense of pasture yiel d, with the peak yield of control pasture being 4 t DM/ha (45%) higher than that of the treated pasture. Improved digestibility in treated grass pastu re was mainly associated with delayed loss of water-soluble carbohydrates ( WSC), except in the seed head. Improved cell wall (neutral detergent fibre) digestibility also contributed to varying degrees and was the main factor contributing to improved digestibility in the seed head. The major WSC pres ent in the treated pasture was fructan. The crude protein concentrations of all plant parts of the senescent pasture, except the leaf blades, were als o increased by treatment with glyphosate.