There is a lot of discussion about protein now at harvest. This is even more punctuated by the discount schedules on wheat protein at some elevators. To understand percent protein in the wheat kernel at harvest, it important to understand how protein gets into the wheat kernel.
The first thing that moves into a developing kernel is protein. It is used to form outer structures of the wheat kernel and the embryo (germinates the seed). This protein is formed from nitrogen moved from the leaves and stems of the plant. The second thing to move into the wheat kernel is the carbohydrates. They are moved into the starchy area of the wheat kernel (the endosperm). This is the part of the kernel that feeds the developing plant after it germinates and before roots begin functioning.
A great deal of the protein in the kernel is deposited into the developing kernel very early in the grainfill process. Over half of the protein is put into the kernel in the first 20 days after flowering. After the initial structures of the kernel are formed, carbohydrates begin moving into the kernel. Then, if the plant is not stressed from weather conditions/diseases/etc during grainfill, the plant will continue to add carbohydrates to the developing seed's endosperm. The additional carbohydrates during a good grainfill period is how we add additional weight to wheat kernels, resulting in higher yields.
There is also protein deposited in the endosperm. However, since protein flows into this area later in the development, it is generally in much less concentration (especially if the plant is lacking nitrogen).
Now, you really can't discuss protein percentages without discussing wheat yields. Protein measured at the elevator is just the percentage of the kernel that is protein. If you have high yields, you likely have more carbohydrates in the kernel. This results in a lower percent of the kernel being protein. If you have lower yields (likely because of plant stress), the kernel has less carbohydrates and a higher percent of the kernel is protein.
The percent protein in a wheat kernel can be influenced by your fertility program. Greater amounts of nitrogen can help minimize nitrogen stress and result in higher protein content (and many times, higher yields)
Can protein content of wheat change after it is ready to harvest?
No, environmental factors cannot change the percent protein in a wheat kernel after the plant senesces (turns golden color). The amount of protein and carbohydrates are set in a wheat kernel at that time.
Rain can change the test weight of wheat, but cannot influence the protein percentage of a kernel.
How does rain affect wheat test weight?
Rain that occurs after wheat has dried down for harvest can change the test weight. That is because as it rains/mists, the wheat kernel will absorb the moisture. This causes the seed to swell slightly. As it dries back out, the seed coat (outer layer of the seed) will not shrink back to the same size and shape. This results in tiny wrinkles in the seed coat. Because of these wrinkles, the kernels will not slide past each other as easily or pack as tightly into a container.
Picture this: If a container holds 500 wheat kernels before the rain. It may only hold 475 kernels after the rain. With less kernels, the test weight (pounds/bushel) will be less than before. However, the protein content will remain the same as before the rain because it is just a percent of the kernel weight that is protein.
All in all, Mother Nature is a big determining factor in the protein content of our wheat. Stress during grainfill does greatly influence our yields and therefore our protein percentage. We can also play a role in determining percent protein by the nitrogen applied before and during the growing season. Finally, variety can play a role in the percent protein, but many times Mother Nature is a much bigger determining factor.