The Power of N

A Look at the Effect of Nitrogen on Wheat Yields and Protein

By David Boehm

Spring wheat growers in the Northern Plains understand the importance of protein content in wheat.  Along with yield, protein is a market factor, and price premiums can often be attained with wheat that meets certain protein standards. Thus, it is often very beneficial to manage wheat fields in an effort to increase grain protein content. 

Although the genetic makeup of varieties as well as environmental conditions can affect protein content, one way to increase protein is through extra or late season nitrogen fertilizer application. 

It is generally accepted that nitrogen available later in the growing season is no longer used by the plant for growth and is made available for storage in the wheat kernels, thus increasing grain protein.  This excess nitrogen may be remaining from pre-plant or late season fertilizers.  “Late season nitrogen has increased protein in many studies,” says Bradford Brown, a University of Idaho crop scientist.  According to Brown, “wheat protein is directly related to the available nitrogen, which is how much nitrogen is required to produce both high yields and acceptable protein.” 

Russ Karow, an Oregon State University crop scientist, has also reported yield and protein increases with early and late season nitrogen treatments.  As well, research at North Dakota State University in 2000 and 2001 studied the effect of nitrogen fertilizer, and found that a significant increase in grain protein is possible. 

The NDSU study, by wheat breeder Bill Berzonsky and research assistant David Boehm, found an increase in protein from 15.2 to 16.2% with nitrogen treatments applied as dry urea at planting (see graph-ic). A trend in increased yield was observed, although it was not scientifically significant, and kernel hardness was increased while test weight decreased with treatments.

According to Jirg Blumenthal, University of Nebraska crop scientist, most of the nitrogen is taken up in the wheat plant by the flowering stage, known as anthesis. He found that 80% of the nitrogen is assimilated at anthesis, and that only 20% is assimilated after this. Providing extra nitrogen at this time may be used as a “bonus.” Once in the plant, the nitrogen generally remains in the leaves and roots until after the kernels develop and start to fill.  Because of this, Brown reported a correlation between nitrogen in the flag leaf at the milk stage and the final grain protein content. Brown suggests that flag leaf nitrogen can be increased by late applications of nitrogen fertilizer, thus potentially increasing final grain protein.

Along with grain protein, Brown also reported pre-plant nitrogen fertilizers increase tillers and kernel numbers per plant, both yield factors. A University of Arizona study by Mike Ottman, also found late-season applications increased protein and hard vitreous amber count in durum wheat.  

At the symposium “Wheat Protein Enhancement with Nitrogen Intervention,” held during the annual meeting of the American Society of Agronomy, Crop Science Society of America, and the Soil Science Society of America last fall,  researchers also reported on bread quality effects due to higher grain protein and nitrogen fertilization.  These increases in bread quality are attributed to protein quality, in relation to equal levels of protein content.

Christina Rawluk, Agriculture and Agri-Food Canada, Manitoba, said that “what benefits protein content tends to also benefit protein quality.” Two flour characteristics used to determine protein quality include water absorption and bread loaf volume. According to Rawluk, flour protein increased with nitrogen rates, and with an increase from 0 - 134 lbs. N, the water absorption capacity of flour increased. 

Rawluk added that the stability of the dough—its resistance to breaking down during prolonged mixing—also increased with treatments. Blumenthal at the University of Nebraska further reported an increase in protein content by fertilization led to higher dough strength and quality. The NDSU study by Berzonsky and Boehm also reported increases in both water absorption and loaf volume with their nitrogen treatments. 

Oregon State’s Karow, who also reported an increase in loaf volume, found that applying nitrogen all at tillering rather than in split applications was better for yield and protein, and that no difference in loaf volume was observed when comparing the time of application.

Environmental conditions during late season fertilizer applications may affect the effort to increase protein. Cynthia Grant, Agriculture and Agri-Food Canada, said that under dry conditions, late season nitrogen applied to the soil may not be available to the plant, while foliar applications are more available.

Generally, researchers believe droughts during grain fill reduce yields by not allowing the plant to store carbohydrates. This may mean that even if test weight and yield are reduced, the percentage of protein in kernels is higher, thus supporting the widely accepted inverse relationship between yield and protein.

Oregon State University extension agronomist Donald Horneck said that adequate rains during grain fill help the plant store carbohydrates and thus increase yield, but these late-season rains may make achieving higher protein difficult, even if wheat growers in Oregon manage for increased protein. Brown added that “at low nitrogen levels, protein increased with yield increases, suggesting perhaps that nitrogen was still limited in the system.  Otherwise, the higher the yield, the lower the protein.”

As with any investment, economic returns must be realized if the efforts of late-season N applications are to be successful. Brown of the University of Idaho suggested that if price premiums for higher protein and discounts for lower protein wheat change, farmers might not always get a return for their fertilizer costs.  He added that hard spring wheat farmers in the Pacific Northwest may be discounted by $1.00/bu. for wheat that does not reach 14% protein. Grant of Agriculture Canada suggested applying nitrogen at seeding due to the unpredictability of the growing season, and adding nitrogen later in the year if higher protein is the goal and yields are expected to be high.

Grain protein content and yield as affected by increasing N treatments in 2000 and 2001, averaged across varieties and locations. Boehm and Berzonsky, NDSU, unpublished data.

Starch Synthesis Key Factor in Inverse Protein/Yield Relationship
It is generally accepted that an inverse relationship exists between wheat yield and protein, meaning if yield is high, protein will be low and if protein is high, yield will be low.  Since both are price factors, it is important to know why it is difficult to get both increased yields and protein in order to maximize economic return per acre.

First, differences exist in the protein levels from various wheat varieties. Weather and fertilization also play key roles in determining yield and protein.   R. Carl Hoseney, a crop scientist at Kansas State University, suggests that protein is synthesized throughout grainfill while starch synthesis starts at the physiological maturity and increases during maturity.  If adequate moisture and nutrients are available during grain fill, starch content will increase, resulting in a higher yield but a lower (by percentage) protein content. Conversely, during dry conditions, starch synthesis will be lower and protein percentage will be higher. Conditions such as frost damage and disease pressure can also decrease the depositions of starch and result in increased protein content and reduced yield.

Nitrogen available to the plant also determines yield and protein. Early nitrogen sources help plant establishment, growth, and tillering, which affects yield. Nitrogen available at flowering can aid in protein deposition. According to Bradford Brown, University of Idaho, “at low nitrogen levels, protein increased with yield increases, suggesting perhaps that nitrogen was still limited in the system.  Otherwise, the higher the yield, the lower the protein.”

The Difference Between Wheat Protein Content and Quality
It is a common misconception that wheat protein content also reflects protein quality. However, while these two components do interact to influence end-use quality characteristics such as dough structure, mixing tolerance, and water absorption, they are independent concepts.

Protein content refers to the quantity of protein in the grain, regardless of its type or nature.  Protein quality refers to the nature of the proteins, their structure, and how they interact with other proteins.  While protein content is highly influenced by the growing environment, protein quality is mostly genetically determined.  Different wheat varieties grown in the same location may have the same protein content, but very different protein quality.

Within a market class, the wheat market treats all wheat varieties with similar protein contents the same, and assumes that they will have the same end-use quality. Unfortunately, this assumption results in a lack of consistency in baking performance and frustration for the end-user.  While protein content is certainly important in determining quality, historical data indicates that the variety itself has much more of an influence on end-use quality. It is ironic that while concerns about end-use quality are often raised when protein content is too high or too low, commercial wheat shipments blended by varieties from the same market class but with very different quality attributes receives much less attention.

While it is the responsibility of wheat breeders and cereal chemists to only release wheat varieties that are superior for all end-use quality attributes, the greatest responsibility for influencing the quality of U.S. wheat lies with the grower.  It behooves the grower to understand that not all varieties are created equal, and that protein content and market class do not necessarily reflect end-use performance. By only growing varieties with superior end-use quality, the growers assures that U.S. wheat shipments will be of the highest quality, regardless of which quality component is used to grade them. 

– From a summary of the report, “The Relationship Between Protein Content and Protein Quality,” by Brady Carter, Kim Kidwell, Stephen Jones, and Tracy Harris, Department of Crop and Soil Sciences, Washington State University, Pullman.