In a winter period field experiment at Redland Bay, SE Queensland, five contrasting barley (Hordeum vulgare L.) cultivars, one malting (Grimmett) and four feed (Cameo, Gilbert, Skiff and Kaputar) plus one wheat (Triticum aestivum) cultivar (Hartog) were grown in two trials with two rates of nitrogen: 0 and 100 kg N ha-1. This experiment was conducted in order to determine the effects of cultivar and nitrogen levels on grain yield and grain protein under two moisture conditions: terminal stress and fully irrigated. In the water stress trials a rain out shelter was used to exclude rainfall on plots from 43 days after sowing to maturity.
Grain yields significantly increased with N treatment and water supply. The mean grain yield averaged over the N rates significantly increased from 3.98 to 4.93 t ha-1 in the stressed trial and from 6.3 to 7.2 ha-1 in the fully irrigated control. N application and ample moisture significantly increased fertile tiller number and grain number m-2 but had no significant effect on grain size. Under water limiting conditions and adequate N the grain size reduced significantly. Significant cultivar differences in grain size, grain number and fertile tiller number were observed.
All differences in grain yield were due to differences in N uptake of cultivars. Hence Kaputar which was able to extract more N than other cultivars in both high and low moisture conditions in the 0 N treatments was able to maintain high yields. Hartog wheat was only able to extract large amounts of N when N was applied and hence had low yields in 0 N and water limiting conditions. Adequate moisture supply significantly reduced the nitrogen use efficiency (NUE) when nitrogen was limiting, but significantly increased it when N was not limiting.
Nitrogen application increased percent grain crude protein (GCP) but the effects of N treatments varied among moisture levels and cultivars. With ample moisture supply GCP decreased with N application, but increased when N was limiting; while grain N and total N uptake increased.
Cultivar differences in GCP were only observed when the crop was subjected to water stress and were related to differences in NHI and grain yield but not due to differences in N uptake and grain N.
The relationship between grain yield and GCP varied with genotype, nitrogen and water status. GCP was found to be negatively correlated to grain yield among cultivars but the effect of N was positive therefore the overall effect became positive when the N treatment effect predominated the negative cultivar effects under low moisture conditions. However with adequate moisture, the negative cultivar effect predominated the positive N effect. A higher correlation between grain yield and GCP was found to exist when nitrogen was limiting with and without adequate moisture, however with N application under water limited conditions, there was no significant relationship between grain yield and grain protein.
Cultivars higher in grain yield tended to be lower in grain crude protein but higher in grain N, and total N uptake than those with lower grain yields. Grain yield only responded positively to NUE under high moisture levels. Differences in grain crude protein among cultivars and differences due to N application were greater at low than at high moisture levels.
N had no significant effect on mean osmotic potential (OP) and relative water content. The leaf water potential however declined faster in the high N than in the low N treatment. For the same leaf water potential cultivars were found not to differ significantly in their capacity for osmotic adjustment within and across N levels.