For the most part, we have had a relatively cool and wet summer across Arkansas that has allowed us to reduce the number of irrigations on corn and grain sorghum. Driving around the state, it is hard to tell much difference between non irrigated corn and grain sorghum fields and fully irrigated fields. As we get closer to physiological maturity many producers are shutting down irrigation wells or will be in the future. Questions are now being asked, when I can stop irrigation and not suffer any yield loss and how long will it take me to get to maturity.
Many times we think that once we make it to the dent stage (R5) in corn (Figure 1.) that we are almost to maturity. I would agree that we are on the downhill slope once we reach the dent stage, but still have a ways to go before maturity.
At the beginning of R5 only about 50% of the total kernel dry weight has been accumulated, so in other words it is very important to have adequate water to maturity for maximum kernel weight. In general, with normal July temperatures, at the beginning of R5 (just beginning to dent), you are approximately 21 days from maturity. The starch line will begin at the top of the kernel (ie, dent) and progress slowly down the kernel (Figures 2, 3). Once the starch line has moved half way down the kernel, you are approximately 10-12 days to maturity. For furrow irrigated corn with good soil moisture, irrigation could be terminated if the starch line has moved 50% or more down the kernel. For pivot irrigated fields, the starch line needs to be 75% or greater before irrigation can be terminated. Keep in mind that fields may be variable in maturity, so check several spots in the field to be sure you are getting an accurate measurement.
In grain sorghum, just like with corn, the goal is to maintain good soil moisture until maturity. Grain sorghum irrigation termination can be initiated when 50% of the heads are at hard dough, (approximately when 50% of the head has changed to red color). One of the problems we often have is the lack of uniformity from top of the field to the bottom of the field, which makes termination a little harder to call. Below are some examples of grain sorghum stages.