Scheduling the last few irrigations of the season deserves extra attention because the goal is not only to focus on keeping the crop wet enough to produce optimal yields, but also on using up stored soil water. Leaving the field a little drier at the end of the season will save irrigation costs, decrease leaching losses, improve soil conditions for harvest traffic, and save water for future years. Growers also don’t want to miss out on capturing off-season precipitation. Lowering the soil moisture level to 40% of plant available water in the top four feet will give about 2.4 inches of off-season water storage in sandy soils and about 5.5 inches in silt loam soils without lowering yields.
University of Nebraska Extension irrigation scheduling recommendations encourage irrigators to allow the crop to continue using more and more of the stored soil water starting in August and continuing into September when the crop matures (Figure 1). The recommendation is to lower the soil water level from the usual summer water condition of no less than 50% plant available water in the top three feet of soil to 40% in the top four feet after the R4 stage is reached — dough for corn, and end of pod elongation for soybean. Thus, the stored soil water content should be significantly lower when the crop matures in September than earlier in August.
According to recent data, many fields are missing this opportunity to improve water use. These fields end the season with fairly wet soil with little to no storage room for off-season precipitation. Each year, the Upper Big Blue NRD requires each farmer in six high nitrate zones across the district to use soil water monitoring equipment in at least one irrigated field. The study analyzed data from the fields that used a Watermark system, which includes three sensors placed at different depths to represent the root zone of corn and soybeans, and a data logger to automatically record the data.
Using the 40% recommendation, the data shows many fields are applying more water late in the season than is needed. Some years, a significant rain can cause the soil to be wetter in September, but it is usually due to applying more irrigation water after the middle of August than needed. The data shows that in 2017, 72% of fields were over-irrigated late in the season. Even in the drought year of 2022, 36% of fields were over-irrigated late in the year.
The data did not ask about late-season decision-making, but it could be because we are all creatures of habit. The irrigation routine is set in July when the plants are transpiring at their peak, the days are long, and the temperatures are high. Then, as the daylight hours shorten (by about three hours by Aug. 20) and the temperatures get cooler in late summer, many keep irrigating at July levels even though crop water use for corn has gone from an average of two inches/week at silking to 1.25 inches/week at full dent (Figure 1). Other crops, including soybeans, have a similar dramatic drop in crop water use moving through August and into September.
The advantages to reducing soil water include saving money on pumping costs, leaving room to store the off-season precipitation, improving field conditions for harvest, saving water allocation, and reducing the potential for leaching nutrients like nitrate deeper into the profile. Even if your irrigation costs are only $10/ac-in, each inch reduction is worth about $1,300 for each quarter-section pivot. For example: A field of silt loam soil, if it is left at field capacity, is missing out on 5.5 inches of off-season storage, worth about $7,200 — not even accounting for the leaching loss of nutrients.
For most of Nebraska, adequate precipitation will be received from October through May to refill the soil profile on irrigated fields. The Grand Island area gets about 14.2 inches during this timeframe and even in Scottsbluff, they still receive about 8.6 inches on average. In addition, leaving the soil drier will help reduce harvest delays and compaction because of mud in wetter falls. For fields with water allocations or shallow aquifers, these water savings can be critical during multi-year droughts.
Source: unl.edu
Photo Credit: istock-laughingmango
Categories: Nebraska, Crops