Fertility and Nutrients
In-crop N management for high protein wheat rarely pays
By Bruce Barker
Researcher speaks on research results.
It is a question that comes up whenever wheat prices and protein premiums improve: if the crop is looking good, can I top dress nitrogen (N) to improve the protein?
Rarely, if only a low N rate of 15kg/ha is applied, says Ross McKenzie, an agronomy research scientist with Alberta Agriculture, Food and Rural Development (AAFRD) at Lethbridge. (Note: kg/ha N x 0.89 = lb/ac N).
“There has been a lot of interest in this topic, but generally, a grower would be better off placing his nitrogen at the time of seeding,” says McKenzie. To significantly increase wheat protein, an in-crop application of N at a rate of 30kg/ha to 45kg/ha N would be needed – a rate of 10kg/ha to 15kg/ha N is simply too low to consistently increase grain protein!
McKenzie bases his comments on a large research program that looked at in-crop application of either a foliar urea ammonium nitrate (28-0-0; UAN) or broadcast ammonium nitrate (34-0-0; AN) application to try to manipulate hard red spring wheat grain protein content. There were 26 site years of data from across the Prairies covering Brown, Dark Brown, Black and Gray soils, including 11 in southern Alberta managed by McKenzie, eight in central Alberta with Jill DeMulder, two at Melfort with Dr. Adrian Johnston, and five at Brandon with Dr. Cindy Grant. McKenzie also had three sites at Lethbridge that were irrigated.
In the research, urea was banded at seeding at zero, 60, 75, 120 and 135 kilograms per hectare. At the urea application rates of 60kg/ha and 120kg/ha, N was also applied at tillering, boot flowering stages of the wheat at 15kg/ha. “Fifteen is about the rate that farmers would want to risk,” says McKenzie, explaining why that rate was chosen as a top dress application.
Nitrogen goes to yield first
McKenzie explains that grain protein content is generally unaffected or declines with the first increment of N fertilizer under extremely N deficient conditions as most of the N taken up by the plant goes to increase crop yield. However, it increases rapidly as N availability approaches the amount required for maximum grain yield, with maximum grain protein content generally achieved at levels of N availability much higher than that required for maximum yield (see Figure 1). Since it is impossible to predict the weather, N fertilizer recommendations usually only target average yields with a reasonable grain protein content of approximately 13.5 per cent.
If growing conditions are below average with below average yields, the protein content is generally higher, and if growing conditions are above average with higher yields, the protein is diluted by the yield, resulting in lower grain protein content.
In recent years, several studies have tried to see if foliar application of N could improve protein content under good growing conditions. In the early 1970s, a two year Manitoba study found that foliar applied urea or broadcast AN applied 10 or 11 weeks after seeding at 34kg/ha increased grain protein content in a year with high grain yields, but it did not increase grain protein content in a year with low grain yields.
“We wanted to look at in-crop applications on a wider basis, and to also consider economics, to see if the practice would pay off for producers,” explains McKenzie, who collaborated with Agriculture and Agri-Food Canada at Brandon and Melfort.
In their research, yields maxed out at a N fertilizer rate of 60kg/ha, because background N fertility was relatively high. At about half the sites, the 60kg/ha gave optimum grain protein content. The in-crop application of 15kg/ha N increased grain protein content slightly at only six of 26 sites compared to spring banded N of 60kg/ha or 120kg/ha. However, when the in-crop N application (60kg/ha banded plus 15kg/ha in-crop or 120kg/ha plus 15kg/ha in-crop) was compared to 75kg/ha or 135kg/ha applied at seeding, only one site had increased grain protein content. This comparison showed that the increase in grain protein content was coming from the increased fertilizer rate, not the split application.
“We weren’t seeing a response very frequently, especially if you considered the overall amount of N applied, either all at the same time at seeding, or split applied with an in-crop application,” explains McKenzie.
Overall, the study found a slight increase in grain protein content of 0.3 per cent. The average increase at tillering was 0.2 per cent, 0.3 per cent at the boot stage and 0.4 per cent at flowering.
Comparing application methods, at three of 26 sites, foliar applied UAN provided a greater increase in protein content, one of 26 sites broadcast AN had a higher increase, and the remaining 22 sites showed no difference between in-crop application methods.
“Direct absorption of foliar N only accounts for about five per cent of nitrogen uptake. The other 95 per cent is washed into the soil and taken up by the roots,” says McKenzie, in reference to why foliar application did not show much advantage over broadcast AN. This means if 20kg/ha N are foliar applied, only 1kg/ha enters the plant through the foliage and the remaining N must be taken up by the roots. The amount of uptake will depend on rainfall to move the fertilizer into the soil, and warm moist environmental conditions for soil bacteria to convert the N fertilizer to nitrate for plant uptake.
Based on 2005/06 prices, application of the additional 15kg/ha at flowering only provided a positive economic return at five of 26 sites. On average, the cost of the additional N was $4.00 more than the increased value of the grain produced – in other words, in-crop N lost $4.00 per acre.
“In-crop application of 15kg/ha did not economically increase grain protein content at most sites in this study,” says McKenzie. “We had a wide range of soil types and growing conditions, so the results are fairly applicable across the Prairies.”
McKenzie explains that in this study, the high soil fertility, as indicated by the 60kg/ha fertilizer rate providing maximum yield at most sites, was one of the main factors responsible for the small increase in grain protein content. High levels of available N reduce the benefit of in-crop N application, and also reduce the efficiency of fertilizer use as maximum yield is approached. In-crop N applications may be more effective if applied under more N deficient conditions.
A final caution from McKenzie is to be wary of recommendations to apply foliar in-crop low analysis N-P-K plus micro blends, which are promoted as a way to ‘boost crop yield’. These yield boosting products are applied at very low rates in the range of five kilograms of product per hectare. He says there is little research in the scientific literature to support these claims, and that growers and agronomists should ask to see local scientific research to support the product claims.
“Very little product is put on, the products are very expensive per pound of nutrient applied, and the benefit will be very, very limited – if there is any benefit at all.”
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