Genetic variation in the feeding value of cereal straw

Abstract

Modern cereal varieties tend to have higher grain yields, improved harvest indices, be shorter and earlier maturing than their traditional counterparts. Since neither higher grain yields nor harvest indices appear to be consistently associated with reduced straw digestibility, the potential exists for breeding or selecting varieties combining these characteristics. Shorter varieties of barley and wheat contain higher proportions of leaf blade in the straw than taller material. In rice, conversely, more leaf blade is found in taller varieties. In barley and wheat, leaf blade is more digestible in vitro than either leaf sheath and stem and thus, straw from shorter varieties is more digestible. In rice, leaf blade is less digestible than the stem fraction. Thus, in rice, as well as barley and wheat, shorter material is likely to result in higher feeding value. Stem diameter does not influence straw quality in barley. A greater number of days from planting to either flowering or grain maturity results in increased leaf-blade content in barley and rice straw. In barley, selection for early maturity is likely to lead to reduced straw digestibility but in rice, straw digestibility could be expected to improve. Feeding trials suggest that early maturity in barley results in lower straw voluntary intake. Proportions of leaf and stem appear to explain variations in cellulase solubility of straw from different barley genotypes more closely than levels of soluble phenolics, residual phenolics or titratable groups. However, levels of soluble phenolics or titratable groups were effective in differentiating between untreated straw and straw treated with sodium hydroxide. The effect of variation in straw morphology on the feasibility of chemical treatment to improve straw quality is discussed. In barley straw, leaf sheath responded more to sodium-hydroxide treatment than other straw fractions, suggesting that varieties with greater proportions of leaf blade and stem are likely to show poorer responses to treatment. Aqueous ammonia had only a small effect on the cellulase solubility of leaf blade, compared with sodium-hydroxide treatment. Straw from varieties with a high leaf-blade content and good initial digestibility is likely to show a smaller improvement in digestibility upon treatment with aqueous ammonia compared with straw from varieties with lower initial value.