‘Do I need to irrigate again?’: Late-season irrigation considerations

By: Vasudha Sharma, Extension Irrigation Specialist

For many Minnesota farmers, this growing season has been hot and dry with limited rainfall. While rainfall has varied considerably across the state, some areas remain dry or very dry as we approach the end of August. As corn and soybean crops move toward maturity, one question I am hearing is: Do I need to irrigate again?

The answer depends on three things:

1. How much water remains in the root zone,
2. Where the crop is in its growth stage, and
3. How much water the crop is likely to use before maturity.

For irrigated fields, these factors can help determine whether another irrigation is needed and when to make the final irrigation.

Don't stop thinking about water just because harvest is approaching

Corn and soybean water use declines as crops approach maturity, but crops can still require significant amounts of water during the later reproductive stages. For corn, water stress during grain filling can reduce kernel weight and ultimately yield (Denmead and Shaw, 1960). In soybeans, moisture availability during pod and seed development remains important for maintaining yield. Water stress during these stages can reduce soybean yield, with the pod-filling period being particularly important for achieving maximum yield (Doss et al. 1974).

This is especially important this year because some Minnesota fields have already experienced moisture stress during critical reproductive stages. If the crop still has significant water demand and there is not enough water in the root zone, late-season irrigation may still provide a yield benefit.

At the same time, a dry summer does not necessarily mean that every field needs another irrigation. Soil water conditions can vary substantially with soil type, rainfall, rooting depth, and previous irrigation. A field that received a timely rainfall may have considerably more available water than a field only a few miles away. Before making another irrigation decision, check the soil and not just the weather.

Soil moisture sensors, soil probing and irrigation scheduling tools can help determine how much available water remains in the root zone.

Consider the crop's remaining water requirement and growth stage

As the crop gets closer to maturity, the less benefit there is from adding water. University of Minnesota guidance provides estimates of the amount of water corn and soybean crops may need from different growth stages through maturity.

For example, corn at beginning dent has about 2 inches of water use remaining through maturity, compared with about 0.8 inches at the half milk line. Similarly, soybean water requirements decline from about 2.9 inches at beginning seed (R5) to about 1.2 inches at full seed (R6).

This means the same soil moisture condition can lead to different irrigation decisions depending on crop growth stage. An irrigation that may be justified during active grain or seed development may provide little economic benefit once the crop is very close to maturity. The goal is not to keep the soil profile full until harvest. The goal is to provide enough water to meet the crop's remaining needs.

Be careful about irrigating ahead of rainfall

This is especially important during a dry year. When rainfall has been limited for weeks, it is easy to focus on the next irrigation opportunity and overlook the possibility of rainfall arriving before the crop reaches maturity. Applying excessive water late in the season can move water and potentially nitrate below the crop's active root zone. As crop nitrogen uptake declines toward maturity, minimizing unnecessary water movement through the soil profile becomes increasingly important.

Check the soil moisture, crop growth stage and short-term weather forecast together before making the final irrigation decisions of the season.

What should Minnesota growers do now?

Consider these questions before making the next irrigation or harvest decision:

1. How much water remains in the crop's root zone?
2. What growth stage is the crop in?
3. How much water does the crop realistically need to reach maturity?
4. What is the expected weather over the next several days?
5. Could another irrigation delay maturity or increase the risk of water moving below the root zone?

A dry season requires careful management, but more water is not always the answer late in the season. The best decisions come from combining information about soil moisture, crop growth stage, weather and expected water use.

References

Denmead, O.T., and R.H. Shaw. 1960. The Effects of Soil Moisture Stress at Different Stages of Growth on the Development and Yield of Corn. Agronomy Journal 52:272–274.

Doss, B. D., Pearson, R. W., & Rogers, H. T. (1974). Effect of Soil Water Stress at Various Growth Stages on Soybean Yield1. Agronomy Journal, 66(2), 297-299.

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