Differences in response of winter cereal varieties to applied nitrogen in the field II. Some factors associated with differences in response

Abstract

Some indices of nitrogen use efficiency of a relatively large number of varieties were measured under field conditions. The reliability of the estimates depended not only on the field and analytical technique employed but on the necessity to measure responses over more than one season. The influence of management and seasonal factors was demonstrated in the first paper of this series.

One important finding of the experiments was that some varieties are capable of yielding more under low fertility conditions whilst others are capable of responding more to higher levels of nitrogen fertilizer. There was in general, a negative correlation between yield at zero applied nitrogen and agronomic efficiency (grain yield per unit of applied nitrogen). It is postulated that some varieties that are capable of efficient utilization of soil nitrogen may be less efficient users of applied nitrogen.

Yield at zero applied nitrogen was only correlated significantly with maximum yield in one of the experiments reported. This suggests that varieties that had a high yield potential did not necessarily express it at all levels of nitrogen. In addition, there was only one significant, and in that case negative, correlation between agronomic efficiency of nitrogen use and maximum yield. Efficiency of nitrogen use was thus not consistently related to yield potential.

Agronomic efficiency was related both to uptake efficiency (total nitrogen uptake/nitrogen applied) and to utilization efficiency (grain yield per unit of nitrogen uptake) but more strongly to the former. It is calculated that agronomic efficiency in winter cereals can be doubled (from 20–30 to about 50 g grain/g nitrogen applied) if the combination of high uptake and utilization efficiencies measured in the experiments can be achieved by plant breeding.

It was found that whilst there was a reasonably consistent positive relationship between nitrogen harvest index and harvest index, the former was not consistently related to utilization efficiency in these experiments. This inferst that translocation of an increased proportion of total plant nitrogen to the grain did not necessarily result in increased grain production per unit of nitrogen taken up.

There was a significant negative correlation between utilization efficiency and grain percentage in two experiments on durum wheat but not in bread wheat or triticale. If minimum grain protein contents are to be maintained whilst utilization efficiency is increased, parental lines which do not exhibit such a negative correlation should be chosen.