Managing for Spring Cover Crop Moisture Consumption Starts in the Fall

October 2, 2023

Zach Larson

Summary: Cover crops can positively or negatively impact soil moisture of the following cash crop. Management decisions starting in the fall can help to mitigate potential issues come spring.

When it comes to water usage in tilled and no-till systems, cover crops can be a double-edged sword. While residue from cover crops and no-till practices can reduce soil evaporation and increase porosity and infiltration, water is ultimately needed to grow a cover crop. And when water consumption outpaces winter and spring rainfall additions, as many parts of the country experienced in 2023, cover crops can negatively impact the following cash crop. However, no-till and cover-cropping systems can be manipulated to reduce spring water consumption, with many management impacts occurring before the first seed is planted. Therefore, taking the time to think ahead and plan for spring water consumption will better your chances of minimizing negative outcomes if dry conditions are present the following spring.

Evapotranspiration (ET) is the loss of water from evaporation at the soil surface and through the transfer of water from the soil, through the plant, and into the atmosphere. The primary drivers of evaporation at the soil surface are temperature and solar radiation, with lesser effects from air movement (wind) and the humidity of the surrounding air. Cover crops and no-tillage directly impact the two main factors through the addition and conservation of crop residues, as residue reduces the temperature and sunlight hitting the soil surface. The ability of the residue to reduce evaporation is closely related to the thickness of the residue mat, with ½ in of residue reducing evaporation by roughly 50 percent.1 Additionally, through reducing the formation of crusts due to raindrop impact at the soil surface, a doubling of residue cover has been shown to double soil infiltration rate.2

Relative water use per crop growth stage
Figure 1. Relative water use per crop growth stage. For most crops water usage peaks at flowering and early grain fill.

The quantity of water removal by cover crops is primarily a factor of growth stage (figure 1), with water consumption peaking at flowering and early grain or pod development. This can be advantageous during a wet spring where a cover crop will rapidly remove water from the soil to create suitable planting conditions. In a ‘typical’ spiring in the eastern US, this drying in the spring followed by deposition of residue works well to accelerate planting and conserve water later in the season (figure 2). However, in a dry spring it can make a bad problem worse, impacting early crop growth.

Broadly, cover crops have been shown to increase and decrease soil available water. though some trends can be identified. A 2022 review of studies revealed that, on average, cover crops reduced soil water by approximately 8% in temperate regions, such as the Southeastern US, to 18% in dry and cooler regions, such as the western and northern US.3 However, many outliers can be found. In Nebraska, a study covering three years and 4 sites showed slight to no differences in water usage between cover crops and winter fallow, and a study in Iowa from 2008 to 2014 determined that soil available water was higher when following a cover crop vs a no cover crop control in 6 of the 7 study years.4,5 Additionally, cover crops have been shown to conserve water relative to no cover crops in the latter part of the growing season on cotton on the Texas High Plains.6

.  Soil moisture consumption through the growing season
Figure 2. Soil moisture consumption through the growing season. Cover crops systems may consume soil water in the early spring, but greater infiltration and residue cover can increase soil available water later in the season.

Strategies for conserving soil moisture

There are many ways to manipulate crop residue and cover crop growth to increase plant available water. Many of these strategies begin with management decisions made in the fall. Therefore, if spring moisture availability is a consistent issue in your area, consider the following points to maximize soil available water.

Manage soil residue

Residue cover has been consistently shown to reduce soil water evaporation.1 Therefore, one should prioritize residue management in the fall. Residue should be evenly distributed out of the combine and incorporating residue with tillage should be avoided. Additionally, greater consideration should be given to the moisture-conserving value of straw and crop residue when determining if they should be baled and removed. When combined with residual nutrients and erosion reduction potential, the value of residue in the field may be greater than its feed value, especially in dry regions where spring moisture is limited and evaporation is high.

Adopt a defensive cover cropping strategy

One of the most defensive cover cropping strategies is to utilize winter-terminated cover crops as stand-alone species or the dominant portion of a cover crop mix. In regions where cover crops winterkill, soils can be recharged with rainfall and snowmelt ahead of spring cash crops, while still gaining value from the added residue that will reduce evaporation. The residue from high carbon-to-nitrogen cover crops such as oats will persist longer than crops such radishes. If an adequate growing season is present in the summer (generally after wheat) sudangrass and sorghum-sudangrass also provide large amounts of lasting residue. This strategy could be employed by planting a mix that includes winter-hardy species that mature later, reducing spring water consumption while gaining many of the common benefits from cover crops.

Select the right winter-hardy species

If fall planting windows limit cover cropping to winter-hardy species but spring moisture consumption is a concern, consider utilizing cover crops with lower spring water usage. Information on water usage of cover crops is still scarce and some inferences need to be made from research performed on forage and cash cropping systems. However, trends in research suggests that legumes such as vetch or clovers consume less water than small grains, with typically less than a 10% difference in water use between species.3, 7-10 However, the later flowering date of legumes relative to small grains, such as cereal rye, means that near the time of corn planting small grains may be near maximum water consumption, while legumes will be at a growth stage that consumes less water. When grown prior to corn, legumes also have the added advantage of tying up less soil nitrogen as their residue breaks down.

Some sources additionally indicate that grasses, such as annual ryegrass, use less water than small grains, with barley using less water than wheat.11, 12 Water consumption of rape, one of the few winter-hardy brassicas in many regions, appears to be similar to that of small grains.13 At least for small grains, seeding rate may have little effect on water consumption, as research has shown that reducing wheat seeding rates by 75% only resulted in 10% lower water consumption.14 However, lower seeding rates can still be beneficial, as they can be more forgiving to plant into, especially if wet spring weather forces a later-than-anticipated termination date.

In this field in Ohio, moisture consumed by the cover crop early in the spring likely impacted early growth, as shown by the stunted plants in the foreground, compared to no cover crop in the background.
In this field in Ohio, moisture consumed by the cover crop early in the spring likely impacted early growth, as shown by the stunted plants in the foreground, compared to no cover crop in the background.

Take an adaptive approach to termination timing

The last management tactic, and likely the most impactful, is to adjust termination timing based on the growth of the cover crop. Killing a cover crop early in its growth stage will result in lower overall water consumption. However, this will come at the cost of a thinner residue mat or one that covers a smaller portion of the soil surface, which may result in greater evaporation and lower soil water content later in the growing season.15 The decision to terminate early should be made based on an assessment of soil moisture conditions as well as the expected weather. If soils are dry and little rain is forecasted, the best decision may be to terminate early to conserve moisture. However, if there’s confidence that mid-season rains will come, delaying termination may have little downside.

In terms of water usage, cover crops can be advantageous and detrimental. Understanding when and how cover crops consume and conserve soils moisture is important in managing cover crops and crop residue. Ultimately, taking an adaptive management approach from species selection to termination is one of the most effective ways of avoiding potential issues when it’s time to plant your cash crop.

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