Summer forages under irrigation. 2. Forage composition

Abstract

Summer forages, including Sorghum bicolor hybrids, S. sudanense, S. bicolor x S. sudanense hybrids, perennial sorghum, maize and Pennisetum and Echinochloa millets, were grown on an alkaline clay soil at Trangie, New South Wales. Primary growth was sampled every 3 weeks and the plant components analysed for nitrogen, sulfur, sodium and in vitro dry matter digestibility. Whole plants were analysed for calcium, magnesium, phosphorus, potassium and chloride content. Fresh leaf samples were analysed for potential hydrocyanic acid (HCN) yield. The quality attributes were regressed on time in order to describe accurately the quality of a forage at different times in its growth. Sorghum species, particularly the sweet sorghum hybrids and perennial sorghum, had high HCN potentials (0.15 and 0.14, respectively, at 8 weeks). They had low contents of sulfur, especially in the stem fraction and this decreased with time. Hence the nitrogen:sulfur ratio was always marginally high or, in sweet sorghum hybrids and perennial sorghum, above the recommended optimum of 15:1 for animal nutrition. Furthermore, they were deficient in sodium, particularly in the leaf fraction, while digestibility decreased rapidly with age. Nevertheless, Piper Sudangrass had a low HCN potential, which would allow it to be grazed early when digestibility was high (70% at 7 weeks). Maize had a negligible HCN potential, a nitrogen: sulfur ratio consistently below 15: 1 and a sodium content intermediate between sorghum and millet species. Millets had a negligible HCN potential and high sodium content, particularly Echinochloa utilis. This species also had a high sulfur content and hence very low nitrogen: sulfur ratio; it is recommended for early grazing to obtain high quality forage.