Proper fertilization affects the efficiency with which bermudagrass uses rainfall during the growing season. Wadell Altom has addressed this topic in News and Views before, but I would like to expand on it. I have taken the data he used and added several more years to paint a more vivid picture.
The data comes from fertilizer response studies that started in the 1970s and continue through today. Experimental sites include areas east and northwest of Ardmore and south of Burneyville on the Red River. The "zero nitrogen" plots had no nitrogen fertilizer applied, but other nutrients were not limiting. The "adequately fertilized" values represent plots that had 150 to 206 pounds of nitrogen (N) per acre applied annually, depending on the particular test, and other fertilizer nutrients were applied as needed.
Figure 1 shows the inches of rainfall necessary to produce one ton of forage with no N added and then with "adequate" N supplied. As you can see, there is a huge difference. On average, the fertilized plot produced a ton of forage from about nine inches of rain, while it takes more than 20 inches of rain to produce a ton of forage without N. Keep in mind that these numbers are averages over 18 site years, and that these sites do not represent the best land resources in the area.
Some data from a Texas A&M publication support the view that a ton of well-fertilized forage can be produced with as little as four inches of rain. I suspect that this is optimistic for this area, and that the rainfall needed to grow a ton of well-fertilized bermudagrass will be somewhere between four and 12 inches, depending on the particular land resource involved.
If the last few summers have taught us anything, it's that rainfall often limits bermudagrass production in this part of the country. One easy way to increase the efficiency of the rainfall received is to fertilize with adequate rates of N.
Something that came up as a side note to this topic is whether or not there is a point of diminishing returns to rainfall. The thought occurred to me that at any given time in the growing season (the month of May, for instance), there might be a point at which yield no longer increases with increasing rainfall.
Figure 2 represents data from the 18 site years cited earlier, but it is focused specifically on May. Over the course of these studies, rain in May varied from less than one inch to more than eight inches. What was important to me was that total season yields were still increasing even when we received eight inches of rain in May. Another thing to note is that adequate fertilizer increased yields at both low and high levels of May rainfall and that as rainfall amounts increased, the yield increase due to fertilizer was even greater.
The graph shows 1.3 tons/acre more forage, when fertilizer was applied, at about one inch of rainfall and 1.9 tons more at slightly over eight inches of rain at the same N rate. This evidence shows that nitrogen is a valuable tool for producers seeking to take advantage of the rain they receive in both wet and dry summers.