Manipulation of the feeding value of senescing annual ryegrass (Lolium rigidum Gaud.) with imazethapyr and glyphosate

Abstract

The effect of applying imazethapyr [(RS)-5-ethyl-2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin- 2-y1)nicotinic acid] on the in vitro dry matter digestibility (IVDMD) of annual ryegrass was studied and compared with the effects of glyphosate [N-(phosphonomethyl)glycine] . Application of imazethapyr 20 days before anthesis delayed loss of digestible dry matter (DDM) more effectively than when applied at anthesis or 8 days after anthesis. Digestibility of the stems of ryegrass when measured at anthesis was 57.9% and 61.9% for 50 g (a.i.) ha-1 and 100 g (a.i.) ha-1 of imazethapyr, respectively, compared with 50.4% for the control. Application of either imazethapyr or glyphosate decreased dry matter yield initially, mainly because growth of seed heads was reduced. Both glyphosate- and imazethapyr-treated plants produced secondary tillers. In the case of the imazethapyr-treated plants, yield forgone as a result of herbicide treatment was recovered through the production of secondary tillers and because seed head yield were not suppressed to the same extent as in the gylphosate-treated plants. When applied 20 days before anthesis, both herbicides maintained stem IVDMD at 54-60% for up to 42 days after spraying, whereas IVDMD of stems in the control declined. The IVDMD of leaf sheaths was intermediate between that of stems and leaf blades and was improved by herbicide treatment. The IVDMD of leaf blades was unaffected by herbicide treatment. Herbicide treatments 20 days before anthesis therefore delayed loss of DDWI from the vegetative part of the sward for up to 22 days after anthesis. By comparison, in the untreated sward, DDM was lost at a rate of 34.4 kg per ha per day. The total DDM of swards (including seed heads) in all treatments did not differ.