Estimates were made of the dry matter (DM) yield and N content of cereal grains harvested from trial plots. They included 236 combinations of variety and treatment, with 13 varieties of winter wheat grown under six husbandry regimes, 14 varieties of spring barley under three husbandry regimes and ten varieties of winter barley under 11 husbandry regimes. The husbandry included timing and rate of fertilizer application, use of growth regulator and fungicide, and seed rate.
For all three cereals, there were highly significant positive associations between grain N content (g/kg DM) and grain DM yield (t/ha) when individual varieties were grown under different conditions. The regression coefficients were: winter wheat 2·28, spring barley 0·74 and winter barley 1·06. When varieties were compared under constant husbandry, the association between N content and yield was negative; the regression coefficients were: winter wheat – 1·53, spring barley –1·14 and winter barley –1·21.
The contents of lysine, threonine, isoleucine and valine were examined in 19 samples of winter wheat, 19 of spring barley and 21 of winter barley. Each cereal type included a comparison of varieties under one husbandry regime and a comparison of husbandry regimes in one variety.
The amino acid composition of wheat protein changed little with either variety or treatment. In spring and winter barley, lysine concentration in grain protein (g/16 g N) decreased with increases in grain N which arose from additional fertilizer N. There were similar decreases in threonine, isoleucine and valine in winter barley, but varietal differences in grain N were not associated with significant changes in the amino acid composition of grain protein.
The nutritive values of spring barley, winter barley and winter wheat were compared in digestion and N balance studies in growing pigs. A subset often samples was examined which included, for each cereal type, high- and low-protein varieties, each (except for spring barley) grown with high or low rates of applied fertilizer N.
Apparent digestibility of dry matter measured at the terminal ileum was similar (0·67–0·70) in all three types of cereal but there was a tendency for the DM of high-protein varieties to be digested better than that of low-protein varieties. Over the whole digestive tract, the apparent digestibility of the DM of wheat was higher (0·83) than that of barley (0·75).
The apparent digestion of the starch of all samples was virtually complete (0·98) by the end of the ileum; the remainder was digested in the large intestine.
The apparent digestibility of N up to the terminal ileum was significantly higher for wheat than for barley, and significantly higher in high- than in low-protein varieties. Over the whole gastro-intestinal tract, the difference between the cereals was even larger; a greater proportion of wheat N than of barley N was digested in the large intestine.
The rate of N fertilizer application did not significantly affect the apparent digestibility of any amino acid but there were significant differences amongst the cereal types in the apparent digestibility of seven amino acids. The amino acids in high-protein varieties were, on average, 6 % more digestible than those in low-protein varieties.
When allowance was made for the endogenous flow of amino acids, some of the differences between cereals and between high- and low-protein varieties disappeared but some significant differences remained.
The biological value (BV) of wheat protein (0·43) was significantly lower than that of barley protein (mean 0·57). When pigs were fed wheat as the sole protein source, the higher N content of wheat compensated for its lower BV. There were no significant differences in the rates of N retention between pigs fed on wheat or barley.