Mike Ottman
Extension Agronomist
University of Arizona

Several instances of low protein grain have been reported in the Arizona durum crop this year. Low protein grain results in loss of income to the grower due to price discounts. Low protein in durum is primarily related to low nitrogen availability, but is often associated with other factors such as yield, irrigation, soil texture, previous crop, variety, and weather. The affect of these other factors on grain protein may be indirect since most of them also influence nitrogen availability. Factors that may influence grain protein content are explained below.

· Seasonal nitrogen fertilizer rate can affect grain protein content and yield. However, the peak grain protein will be attained at a higher nitrogen rate than the peak grain yield.

· Late season nitrogen fertilizer is more effective than early season applications in increasing grain protein content. The optimal timing of late season nitrogen to increase grain protein content is between flowering and 2 weeks thereafter. An application of 38 lbs N/acre will increase grain protein content by about 1%.

· Nitrogen fertilization efficiency can affect grain protein content. Nitrogen applied in the irrigation water is only applied as efficiently as the water is applied. Anhydrous ammonia injected in high pH irrigation water is particularly subject to loss. Broadcast urea and ammonium fertilizers are subject to volatilization losses before incorporation with irrigation water.

· High grain yields can result in decreased grain protein due to protein dilution. The amount of protein on a per acre basis could be the same for high yields of low protein grain as low yields of high protein grain. In the case of high yields, a given amount of protein may be diluted in a higher amount of starch in the kernel, resulting in a lower protein concentration.

· Frequent irrigations can decrease grain protein due to increased yield or reduced nitrogen availability. Nitrogen availability may be reduced due to waterlogged conditions, whereby lack of oxygen inhibits nitrogen uptake, or due to losses of nitrogen through leaching or denitrification (gaseous loss).

· Coarse textured soils may produce low protein grain since they are likely to have high leaching potential and low nitrogen content. Soil nitrogen usually accounts for about half of the crop nitrogen uptake and fertilizer accounts for the other half.

· The previous crop can influence durum protein content due to the effect on soil nitrogen. Alfalfa increases soil nitrogen available to the next crop. Many vegetable crops are heavily fertilized and result in high levels of residual nitrogen. Some crops such as small grains, corn, and sorghum leave a residue that depletes soil nitrogen during decomposition.

· Certain durum varieties are genetically predisposed to low protein content. However, most durum varieties grown in Arizona are capable of attaining 13% protein if nitrogen availability is sufficiently high.

· The growing season weather appears to influence grain protein content in some years due to the effect on yield. Low protein content seems to be more common in high-yielding years. However, low grain protein was attained this year in some fields that were not particularly high yielding.

Low protein content in durum can be avoided in most cases by being aware of the factors that influence protein content. An application of 30 to 60 lbs N/acre (or more) between flowering and 2 weeks thereafter may be necessary to achieve 13% protein. The lower stem nitrate test is a good indicator of the nitrogen status of the crop and the need for late season nitrogen fertilizer.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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