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Early summer crop water needs crucial for optimal growth

Early summer crop water needs crucial for optimal growth

By Andi Anderson

With corn and soybeans in their rapid growth stages, it's time to prepare for irrigation to match crop water use.

While early summer rainfall may have initially provided soil moisture, factors such as cover crops, tillage, or insufficient rainfall can affect soil moisture conditions, necessitating the start of irrigation soon.

Irrigation for late-season planting after harvested forages like wheat or rye helps crops establish and develop as they enter the typically drier summer weather.

The first or spring crop often depletes soil moisture reserves, requiring irrigation to germinate the second crop. In some cases, irrigation before planting the second crop aids in seedbed formation.

Early irrigation also helps incorporate fertilizer and herbicides or soften soil crusts for better crop emergence.

However, over-irrigation during vegetative stages should be avoided. Excess water in the root zone can lead to deep percolation and leaching, especially in sandy soils, resulting in the loss of agrichemicals.

To determine if crops need irrigation, there are simple tips and tools available. Crops in vegetative stages like corn at V4 to V8 typically use 0.5 to 0.8 inches of water per week, while soybeans at V1 to V2 use 0.4 to 0.6 inches per week under normal conditions.

Crops with a full canopy, such as winter wheat and forages before cutting, use 1.5 to 2 inches per week.

While crop water use during vegetative stages is low, it is essential to monitor water deficits.

Michigan State University Extension encourages irrigators to compensate for the previous week's water use. However, if rain is forecasted, leave space for it to refill the soil profile to prevent runoff or deep percolation.

Irrigation scheduling maximizes efficiency. One method involves using reference potential evapotranspiration (rPET). In southern Michigan, the water used by well-watered grass in late June is about 1.4 inches, though this can vary based on weather conditions.

The Michigan State University (MSU) Enviroweather network provides rPET estimates and related tools for each of its 87 Michigan sites. These data can guide irrigation based on local weather conditions, and the network offers a daily rPET text service to subscribers.

The rPET needs to be adjusted for the specific crop's water demand. The ET for annual crops increases until full canopy is reached. Early-season rooting depth limits irrigation application volumes.

Applications of 0.75 inches or less are common this time of year to avoid pushing water below the effective root zone. However, avoid making too many small (less than 0.4 inch) applications, as they are less effective and can aggravate disease by frequently wetting the plant.

Corn at the V6 stage has an effective rooting depth of 20 inches. At the V10 stage, corn has a 23-inch effective rooting depth, and by the VT (tassel) stage, it reaches full effective rooting depth of 36 inches or more, rapidly reaching peak daily water use rates during pollination.

Soybeans at the V3 stage have an effective rooting depth of 16 inches, extending to almost 24 inches by the R1 stage.

For example, the table below shows estimated crop water use for corn and soybeans at different stages according to weather conditions for three locations. In the last week of June, V10 corn will use approximately 88% of the rPET, while V3 soybeans will use about 70%, or almost 1 inch.

Early-season irrigation can be more accurately scheduled by monitoring soil moisture in the root zone rather than using a checkbook irrigation scheduling system for newly emerged crops.

Soil moisture monitoring helps determine whether the field is dry or wet, guiding when and how much water is needed. Monitoring multiple depths of soil can determine which soil layers are dry or wet.

There are several ways to monitor field soil moisture content. The feel and appearance method is popular and quick.

This method involves taking soil samples using a soil probe, auger, or shovel, then squeezing the samples firmly in your hand to form a shaped ball and comparing it with photographs shown in the United States Department of Agriculture Natural Resources Conservation Service standard.

A reliable method is using soil moisture sensors. These sensors estimate soil moisture content based on either the force required to pull water out of the soil (tension) or the electrical properties of the soil, both of which change based on soil moisture content and type.

Soil moisture data can determine when the soil is getting drier and how much water is stored in the root zone, helping schedule irrigation with more confidence.

Photo Credit: gettyimages-studio2013

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Categories: Michigan, General

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