Foliar-applied nitrogen for grain protein and canopy management of wheat
This review was undertaken to investigate the efficiency of crop capture and recovery of foliar-applied nitrogen (foliar N), assess effects on the crop and the factors influencing the occurrence of crop damage, and review where the industry might benefit from further research and development.
Grain protein levels have shown a tendency to decline in recent years. The advent of high-yielding cultivars with bread-making potential may have exacerbated the problem. Application of foliar N late in the season is the most efficient method of boosting grain protein, but UK growers favour the use of granular N applied earlier in the season, as yield responses are more likely, and there is a risk of leaf scorch with urea N. Yield responses to late season foliar N are more likely where previous soil-applied nitrogen is sub-optimal and where foliar N is applied prior to anthesis. Foliar N applied close to ear emergence can delay leaf senescence, prolonging photosynthesis for about 7 days. The optimum application timing to boost grain protein is typically between anthesis (GS 60-69) and grain milk development (GS 70-79). The extra protein derived appears to be of a similar quality to that obtained from soil-applied nitrogen. The response in grain protein is of the order of 0.8% per 30 kg ha-1 N applied.
Typically 60-70% of the foliar N spray is intercepted by the crop canopy. A significant proportion of the applied spray is deposited on the soil surface. Up to 65-70% of foliar N is usually recovered in the crop, similar to values for soil-applied N, though less nitrogen in total appears to be retained in the soil-crop system at harvest. Loss pathways for foliar N do not appear to be fully characterised, leading to a number of assumptions regarding the loss of foliar N from the crop, soil and plant system. Around 10% of the urea-N applied can be 'lost' via volatilisation of ammonia, compared to 0.8% for ammonium nitrate. Adjuvants can increase uptake of foliar N, but with the risk of increasing leaf scorch. The exact cause of scorch is not clear. Accumulation of urea, ammonium or other interim products of protein synthesis appear to be the most plausible cause. Where scorch does occur, damage can typically affect up to 10% of the sprayed leaves at typical field application rates of 40-60 kg ha-1 N.
In the future, to meet the demands of high yield and grain protein, without increasing lodging or disease risks, more N is likely to be required later in the season especially if retention and crop utilisation can be improved and the problems of scorch overcome or reduced.
A number of areas for further work and development were identified:
1. Development of methods for grain protein prediction.
2. Nitrogen strategies for high yielding milling wheat.
3. Improving crop utilisation of foliar N and minimising the risk of scorch.
4. Studies of wheat with modified rates of urease or glutamine synthetase activity to improve understanding of foliar N loss pathways and the causes of scorch damage.
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