The end of the irrigation season is in sight for many of our early planted corn and grain sorghum fields. For the most part this year we have had to irrigate less than some years due to abundant rainfall, but recent high temperatures and lack of consistent rainfall makes having adequate soil moisture to maturity very important to preserve maximum yield potential. 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.
Corn Irrigation Termination
Once we get to the dent stage (R5) in corn (Figure 1.) we are approaching maturity, but it is a slow process to maturity (black layer) and we will still likely need irrigation to maximize yield potential.
At the beginning of the R5 stage, only about 50% of the total kernel dry weight has been accumulated, so in other words it is very important to have adequate soil moisture to maturity for maximum kernel weight and yield. In general, with normal July temperatures (90’s for highs and 70’s for lows), at the beginning of R5 (just beginning to dent), you are approximately 21-23 days from maturity. The starch line will begin at the top of the kernel and slowly progress 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 and good soil moisture before irrigation termination. Keep in mind that maturity may be variable across a field, so check several spots to be sure you are getting an accurate measurement of maturity. If in doubt whether the plant will have adequate moisture until maturity, it is advisable to irrigate once more, especially if hot and dry conditions are present.
Growing Degree Days (GDD), aka heat units, can also provide an estimate on how close corn is to maturity. Most corn hybrids grown in Arkansas are full season hybrids and require approximately 2800-2950 GDD’s from planting to reach maturity. In looking at GDD’s this year, accumulations are at or slightly above the 30-year average. GDD’s accumulated from March 31th to July 16th across the state range from approximately 2400-2500 in Southeast Arkansas, 2350-2450 in Central Arkansas, and 2200-2350 for Northeast Arkansas. Based on accumulated GDD’s the earliest planted corn in southeast Arkansas should be approaching maturity in 10 days and field observations agree with this. The earliest planted corn in Southeast Arkansas is likely receiving the last irrigation this week, while later planted corn and corn further north may need one or more irrigations to reach maturity if rainfall does not occur.
Grain Sorghum Irrigation Termination:
This year Arkansas grain sorghum acreage is estimated to be 500,000 acres, much higher than in recent years. With increased acreage, many are growing grain sorghum for the first time in many years and are having questions about how quickly grain sorghum will progress to maturity and when irrigation can be terminated. As a general rule of thumb, it generally takes about 40 days from beginning of flowering to get to maturity with the full season hybrids and normal July temperatures and approximately 28 days from the beginning of flowering to hard dough. Of course temperatures and hybrid can cause variation in these estimates and there will be variations in maturity across the field.
In grain sorghum, just like with corn, the goal is to maintain good soil moisture until maturity. Grain sorghum irrigation termination can occur on furrow irrigated fields when 50% of the heads are at the hard dough stage (50% of heads with good color development) and adequate soil moisture is present. For pivot irrigated fields, 75% of the heads should be at the hard dough stage (75% of the heads with good color development) and have adequate moisture before terminating irrigation. One of the problems we often have is the lack of uniformity in maturity across a field, which makes termination a harder decision. With the wet spring we have experienced, many grain sorghum fields have considerable differences in maturity from one side of the field to the next.
Below are some examples of grain sorghum growth stages and irrigation management recommendations for each stage: