Managing Nitrogen to Increase Protein in Grains

Water supply and weather conditions have too long been offered in explanation of much that in reality is soil fertility and plant physiology. This becomes increasingly evident as we better understand the chemical and biochemical dynamics in the soil and the biosynthetic services within the plant creating feed and food compounds for us. Variations in the protein content of wheat and corn are not attributable primarily to plant variety or to the conditions under which the plant is grown. They correspond, in large measure, to variations in the fertility of the soil itself.

The tendency toward higher protein content or “hardness” in wheat as one moves westward through Missouri and Kansas has long been associated with the rainfall factor. Careful study, however, reveals that the fertility of the soil and not the weather’s vagaries or the specific variety is responsible for the concentration of protein in the grain. A high concentration results when a generous supply of nitrogen is delivered by the roots just before the heading of the wheat takes place. Accordingly then for high protein or hard, horny grains, the roots must have access to adequate supplies of available nitrogen at that stage of the plant’s development. If not already there, they can be provided by applications of soluble nitrogen.

That this hypothesis is correct was established by Dr. R. L. Lovvorn, now of the U. S. Department of Agriculture, and Professor M. F. Miller at the University of Missouri. They demonstrated wide variations in the “hardness” or in the concentrations of crude protein in the grains of a single variety of wheat: This was accomplished by successive applications of nitrate fertilizers at intervals of two weeks from April 4 to May 29 on a.series of plots in a field of Putnam silt loam.

On this soil, with its shallow surface layer overlying an acid, tight-clay subsoil, the successive applications of soluble nitrogen gave increasingly higher concentrations of crude protein in the wheat. The kernels were less and less yellow in color and more nearly translucent. They were harder. The percentage of protein stepped up from a minimum of 8.92 from the early applications to a maximum of 17.00 from the later applications. This was an increase of 8.08 percent in the absolute. It was a relative increase of almost 90 percent in the higher over the lower concentration.

Figure I. As the date of nitrogen application on wheat is later in the growth period, the protein as percentage in the grain goes up (Curve A); but the yields as bushels of grain (Curve B) and as pounds of total protein (Curve C) per acre both go down.
Data by R. L. Lovvorn and M. F. Miller, Missouri Agricultural Experiment Station.

 

Here then on acid soil, developed under conditions of heavy rainfall, the variation in the stage of plant growth at which nitrogen was made available to the crop produced as wide a range in the concentration of protein as has been recorded for the entire wheat crop from Missouri to western Kansas. Thus in Missouri, which is regarded as a “soft” wheat state, apparently we can have any kind of wheat and any kind of flour, from the “soft” (biscuit) to the “hard” (light bread) variety, depending upon the fertility of the soil rather than upon the weather or the variety of wheat.

The time of applying nitrogen affects not only the concentration of crude protein in the grain but also the total yield of grain and of protein. Although the late application increased the concentration of protein in the grain, it reduced the yield of both grain and protein. The yields of grain dropped from a maximum of 24.2 to a minimum of 11.7 bushels per acre. Yields of total protein fell from 160 to 91 pounds per acre. These data were for single plots and for different dates of nitrogen application.

Here is the suggestion, then, that for good vegetative growth and large yields of grain, the nitrogen must be available early in the growing season. For a high concentration of protein in the grain, extra nitrogen must be available shortly before heading time. By correlating the nitrogen application to the physiology of the plant so as to determine whether the nitrogen is employed in making vegetative mass or in synthesizing protein in the grain, it should be possible to produce both a large yield of bushels per acre and a high concentration or percent of protein in the grain.

If a strain of wheat grown in Missouri and Kansas showed such variations in concentration of protein in the grain, should they not be attributed to the variation in the stages in the plant’s development at which the roots encounter large amounts of available nitrogen in the soil? If so, then it is logical to believe that the higher concentration of protein in the wheat grain as we travel west from Missouri across Kansas, is to be explained on the ground that the roots find soil strata with more available nitrogen at successively later dates in the plant’s growth period. If that is so, it follows that the delay in making contact with the nitrogen supply would depend upon the depth of the strata in the profile. Making this contact near heading time means a “hard,” horny, or high protein grain.

This reasoning is validated by the studies of Professor J. C. Russell of Nebraska, who found that the smaller amount of soil moisture required for the production of nitrates in contrast to the quantity needed for plant growth permits the nitrates to be made and to move downward in the soil faster than the roots can follow. It should not strain the imagination to visualize the nitrates in the downward movement through this under-developed, mineral-rich, near-Chernozem soil, being overtaken by the wheat roots just about “heading time.” This would naturally mean lower yields in bushels per acre and higher concentrations of protein in the grain. Soils of the more humid region that do not have generous nitrate delivery from the reserve organic matter in a narrow carbon-nitrogen ratio nor the higher inorganic fertility mobilizing it, cannot be expected to have either high-grain yields or high concentrations of proteins in the grain. They do not provide the microbial performances associated with, and required for, such a specific timing of natural delivery of nitrate as occurs in the semi-humid soils farther west.

Figure II. The soil profiles and virgin vegetation across Kansas with rainfall decreasing from 37 to 17 inches on going westward. In the soils are the suggestive reasons for the supply of nitrates migrating downward and for the different times in the plants’ growth when the roots overtake them.

 

If the results of three surveys made in the last 10 or 11 years may be accepted as conclusive, there has been a decline in the protein concentration in Kansas wheat equal in significance to the relationship between the timing of extra supplies of available nitrogen and the protein content of the wheat grain. A survey of the percentage protein of Kansas grain made in 1940 showed a range of 10 to almost 19 percent. In a similar survey ten years later, in 1949, protein concentration was found to range from 9 to less than 15 percent.

During this 10-year period, Kansas produced seven record-breaking wheat crops. One of them was nearly 300 million bushels. Annual removal of so much nitrogen from the soil without adequate replacement suggests the possibility that the supply of nitrogen in the soil may have been reduced to a level at which it is no longer capable of maintaining the high concentration of proteins in the grain of which that state once boasted. If so, attention must be given to the use of commercial nitrogen fertilizer on the wheat crop near heading time if production of the high-protein grain of the past is to be continued.

We do not know of any report of studies of the concentration of crude protein in the corn grain as related to the application of nitrogen at specific periods in the plant’s growth. It is significant, however, that the concentration of protein in the corn grain dropped from 9.5 to 8.5 percent during a 10-year period in which there were substantial increases in the yields of grain. It is interesting to note the reduction in the protein content of corn as reported in successive editions of a standard handbook of feeds and feeding. In the Eleventh Edition, published 40 years ago, the only figure quoted for crude protein of dent corn was 10.3 percent. In the Twenty-First edition, 1950, five grades of corn were cited, for which the protein figures ranged from 8.8 to 7.9, with a mean of nearly 8.4 percent. During the interval of 40 years between the two editions, crude protein in corn dropped from 10.3 to 8.4, a reduction of 22 percent. Thus, it is strongly indicated that the high rate at which nitrogen is being removed from the soil without compensating replacement is adversely affecting the concentration of protein in corn, as well as in wheat.

In view of the declining concentration of crude protein in both the corn and wheat grains, the question arises whether the array of amino acids which compose it is now as complete in terms of the nutritional requirements of animals and man as it was at the higher concentrations of proteins in these feed and food grains. It is logical to expect that nitrogen which is available early in the plant’s growth period will be more completely metabolized into the complex proteins composing the germ. Whether this extra metabolism guarantees the conversion of the nitrogen into the more complete list of essential amino acids remains to be established.

Nitrogen delivered later in the plant’s growing period shows up as protein in the endosperm and distributed as simpler amino acids through the starchy reserve portion of the grain. This suggests the use of the nitrogen almost wholly for making the simpler, more common, amino acid, leucine. In its chemical structure, leucine is similar to starch or to the simpler carbohydrate structure with its six carbon atoms. Thus, one might be inclined to accept the late delivery of nitrogen as the reason for its quick attachment to this common carbon chain by some simple amination process rather than by successive biosynthetic elaborations resulting in a less common amino acid like tryptophane, for example. Compared to other amino acids, leucine is relatively simple in structure. It is widely distributed and is found in the starch rather than in the embryo portion of the seed. It makes up about 25 percent of zein, the incomplete corn protein which lends itself like cellulose to artificial fiber synthesis. For these reasons, this “late” protein in the plant’s biosynthetic processes seems decidedly “crude” as far as its nutritional service to animals is concerned. Leucine is regarded as a conversion of nitrogen into protein but it hardly rates the high nutritional evaluation of the truly complete proteins.

We are experiencing a world shortage of proteins but are slow to recognize the lack of soil fertility adequate for biosynthesis of complete proteins as responsible for it. Instead, our complacency in larger production of “crude” protein, according to ash analysis, by soil treatments, and our dependence on commercial nitrogen for higher yields per acre, are apt to lull us into indifference about the fundamental use of nitrogen via the crop’s biosynthetic services and the soil in relation to other aspects of fertility if we are to be well fed. The time of root contact with nitrogen in the soil by the growing crop, has significance for the quality as well as the quantity of the final food product.