THE FEEDING VALUE OF CEREAL STRAWS FOR SHEEP .1. WHEAT STRAWS

Stubble samples from nine wheat crops (Triticum aestivum cv. Millewa) grown at different locations in Victoria were analysed for in vitro organic matter digestibility (IVOMD). On the basis of this measurement, four of the stubbles were harvested for a feeding trial with sheep. The straws used varied in the proportions of morphological fractions from 0.41 leaf:0.46 stem to 0.20 leaf:0.71 stem. The remainder of the material comprised rachis, weeds and dust. The leaf blade and sheath fractions contained ≤ 905 g organic matter (OM) kg-1 dry matter (DM), while the stems contained ∼ 950 g OM kg-1. Leaf blades contained 650-690 g kg-1 DM ash-free neutral detergent fibre (NDF), leaf sheaths 720-760 g kg-1 and stems 820-880 g kg-1. As a consequence of the higher cell solubles content, leaf blades (0.48-0.54) and sheaths (0.43-0.55) had a higher IVOMD than stems (0.25-0.36). At the same time, the NDF in leaf components was more digestible than that in stem material. In addition to these differences, leaf blades contained more nitrogen (0.57-0.77 g kg-1 DM) than leaf sheaths (0.31-0.63) or stems (0.21-0.37). The leaf blades were also less resistant to physical breakdown with grinding energy values between 70 and 90 J g-1 DM compared with values of 110-130 J g-1 DM for sheaths and 200-230 J g-1 DM for stems. These differences in the proportions and characteristics of the main morphological fractions of the straw were primarily responsible for differences between the four straws offered to sheep. For example, they had IVOMDs from 0.47 down to 0.30 and grinding energies from 140 to 200 J g-1 DM. Eating rate tests indicated differences (P < 0.05) between the straws in their acceptance by sheep; rates of consumption with trained animals varied from 290 to 520 g straw h-1. The OM intake of the straws varied (P < 0.001) from 300 to 620 g day-1 or 7.1 to 13.4 g kg-1 fleece-free liveweight (W). There were corresponding differences in NDF intake, but the in vivo digestibility of OM (0.40-0.43) or NDF (0.39-0.46) did not differ between straws. The digestible OM intake varied (P < 0.001) from 120 to 270 g day-1 or 2.8 to 5.9 g kg-1 W. The sheep spent between 14 and 16 h day-1 chewing during eating and ruminating on straws which were readily consumed, but this fell to 11-13 h day-1 for straws which were poorly eaten. The results indicated that the proportions of morphological fractions in wheat straws markedly affected their nutritive characteristics; straws with more leaf material ranked higher on the basis of commonly used nutritive indices. They were also consumed in greater quantities. The consequences of this in regard to practical feeding of straws and to the value of commonly used indices of feeding value are discussed. © 1990.