Digestibility of senescing annual ryegrass following application of glyphosate

Abstract

The effect of time of spraying glyphosate on a sward of annual ryegrass (Lolium ridigum Gaudin) was examined. Glyphosate (180 g a.i. ha-1) was applied at weekly intervals between 21 days prior to anthesis and 14 days after anthesis. The resulting physiological and chemical changes were monitored to assess the contribution of plant components to digestibility during senesence. When applied before anthesis, glyphosate delayed the loss in digestibility of senescing annual ryegrass. However, glyphosate applied at, or after anthesis did not alter the decline in digestibility of the grass compared to the control (unsprayed) treatment. Glyphosate applied before anthesis significantly reduced the development of the grass ear, and delayed the loss of materials soluble in neutral detergent (NDS) from the stem for up to 35 days after anthesis. In control plants, soluble carbohydrates comprised up to 90% of the NDS in the stems, 66% in leaf sheaths, and 25% in leaf blades but were lost almost entirely during senescence. Glyphosate applications before anthesis delayed the loss of soluble carbohydrates until 35 days after anthesis. However, soluble carbohydrate loss occurred after this period and by 66 days there was little difference between control and treated plants. The IVDMD of the neutral detergent fibre (NDF) declined in all parts of the unsprayed grass during senescence. When plants were sprayed with glyphosate, the digestibility of the NDF was maintained or declined only slowly after spraying. Application of glyphosate prior to anthesis resulted in higher stem and sheath nitrogen concentrations. By contrast, glyphosate had little effect on leaf nitrogen concentration. Conservation of feeding value in senescing grass pasture was achieved only by application of glyphosate before anthesis. However, some losses of soluble carbohydrates, and thus digestibility, could not be avoided.