Water-soluble carbohydrates and in vitro digestibility of annual ryegrass (Lolium rigidum Gaudin) sown at varying densities

Abstract

An experiment is described in which the tiller density of microswards of Lolium rigidum was varied by altering planting density. The treatments were expected to alter the interplant competition for light and thus affect the concentration of water-soluble carbohydrates (WSC) of the grass swards. Swards were grown at planting densities of 100 (low density), 500 (medium density) and 5000 (high density) plants m-2. Mainly because of compensatory tillering in the low and medium density swards, all swards had equivalent dry matter yields by 13-15 days after anthesis. The concentrations of material soluble in neutral detergent (NDS) and of WSC, in particular, varied in response to the treatments, and the yield of WSC in the low density sward was 30-50% higher than in the other swards. The yield of WSC reached a maximum at anthesis in all treatments with large losses of WSC occurring as the grass senesced. Fructans were the major type of soluble carbohydrate in all plant parts (50-99% of WSC). At anthesis, the in vitro dry matter digestibility (IVDMD) of the low density sward was higher than that of the high density sward (60.7% and 54.3% respectively). When dead, whole plant IVDMD had declined to 40.8% in the low density and 37.9% in the high density sward. The higher digestibility of the low density grass sward was a result of higher digestibility in almost all component plant parts. Opportunities may exist to improve the feeding value of senescing grass pastures if mobilization of WSC can be delayed or regulated to ensure higher retention of NDS.