Chemical manipulation of senescing grasses to improve feeding value.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 18 CHEMICAL MANIPULATION OF SENESCING GRASSES TO IMPROVE FEEDING VALUE R.D. ARMSTRONG**, R.J. SIMPSON* and G.R. PEARCE** Chemical treatment of pasture grasses offers a possible method of conserving 'Haydry standing fodder of high feeding value for use during summer. freezing' by treatment of mature herbage with paraquat (Gramoxone, ICI) has been tested widely and results in a dead forage reserve with relatively high protein content but does not maintain the original digestibility of the treated material (Pullman and Allden 1971). This paper reports the effects of chemical treatment of mature annual ryegrass pasture with glyphosate (Roundup, Monsanto). A stand of annual ryegrass (Lolium rigidum Gaud.) growing at Mt. Derrimut, Victoria, was sprayed with 180 g (a.i.) glyphosate/ha one week before anthesis Pasture was harvested at regular intervals and and at anthesis (18.10.88). The plant sorted into stem (excluding ear), leaf sheaths and leaf blades. material was analysed for soluble carbohydrates and in vitro dry matter digestibility (IVDMD). The largest effects were seen in stems which comprised the bulk of plant dry The matter (55% at anthesis) and were also the least digestible plant part. IVDMD of stems in untreated pasture declined from 53% at anthesis to 18%, 2 months later (Fig.l). Time-of-spraying was important with grass treated before The stems of this treatment anthesis retaining higher IVDMD when senescent. Leaf blade IVDMD, by were still 36% digestible, 2 months after anthesis. contrast, was not affected and leaf IVDMD declined from 75% at anthesis to The response of-leaf sheaths to the treatments about 51%, two months later. The effect of spraying on changes in IVDMD were associated was intermediate. with a delay in the loss of soluble carbohydrates provided pasture was treated (Fig. 2). Stems of pasture sprayed before anthesis retained before anthesis 8.3 % (w/w) soluble carbohydrate as opposed to stems of control plants which The retained only 0.5% soluble carbohydrates, 2 months after anthesis. treatments were also successful in retaining relatively high NDF digestibility. Chemical treatment with glyphosate before anthesis retained relatively high IVDMD for at least 3 to 4 weeks after treatment. Thereafter, IVDMD of treated herbage also declined, but this material was still of better feeding value than Only low rates of herbicide application appear untreated senescing herbage. necessary (e.g. Davidson et al, 1988) and this is also an environmentally desirable feature of the forage conservation method. PULLMAN, A-L. and ALLDEN, W.G. (1971). Aust. J. Agric. Res. 22:401. -DAVIDSON, K-J., McGUINNESS, S-F., Simpson, R.J. PEARCE, G.R. and HUDSON,D. (1988). Proc. Aust. Soc. Anim. Prod. 17:387. * School of Agriculture and Forestry, The University of Melbourne, Vic. 3052. +* Department of Primary Industries, Rnerald, Qld. 4720, 448