Cover Crops – Stealing or Conserving Water?

Water use and cover crops can be very simple and complex all at the same time. Cover crops, like all plants, need and use water; however, the complexity can come in the amount of water used, when it is used, and the potential water balance changes with surface evaporation alterations, soil physical and biological changes, snow "catching" ability, etc. It also can be further complicated by environmental conditions and management systems. This article will highlight some research studies demonstrating ways cover crops alter the water balance and how you can use cover crops to consume or conserve water.

The "definition" of too much or too little water in agriculture often changes by crop, time of year, equipment, the forecast, and likely many more. Cover crops can be used to help manage the water to be "just right", but it comes with challenges and risks.

In the typical corn and soybean rotation in the Midwest, an over-wintering cover crop that grows rapidly in the spring is quite common. This rapid spring growth coincides with cash crop planting and the need for enough moisture for germination, but not too much moisture to limit field work. Various strategies, results, and scenarios can take place and impact the water use and availability.

A study by Basche et al. in central Iowa over a six-year period on a winter rye versus no cover crop in a corn-soy rotation found the cover crop increased the field capacity water content by 10-11% and the plant available water by 22-22%. Conversely, Martinez-Feria, et al. showed over a similar period and location that the cover crop rye transpiration ranged from 0.43 to 1.73 inches and showed in the dry years of 2012 & 2013, there were yield penalties and reductions in soil moisture.

A study in a drier environment in far eastern New Mexico from 2018-2020 showed cover crops reduced soil water during the non-cash crop period but increased volumetric water content during the cash crop phase due to the residue cover. Chalise et al. (2018) found in a three-year study in South Dakota on soybeans grown after corn that the soil water infiltration was 80% higher compared to no cover crop comparison and had a soybean yield increase of 14%.

A study in Akron, CO and Sidney, NE in 2012 and 2013 by Nielsen, et. al, showed cover crop water use averaged 1.78 times greater than evaporative water loss to the no-cover, fallow crop area. Interestingly, they also showed there was no difference in the water use of the single-species cover crop vs the multi-species cover crop mix.

These are just a few research examples of the immediate gains or losses that can result from cover crops using too much moisture or providing the necessary changes to store valuable moisture.

Many studies have found the extended length of time (often 10+ years) it takes to increase soil organic carbon (helps absorb moisture) and improve soil structure to increase plant available water and infiltration (Sindelar, et al. 2018). Basche and DeLonge (2017) found that continuous living cover for over 10 years were more likely to have a positive effect on soil water properties, such as porosity and water retained at field capacity. This is not to discourage taking on this challenge, but to illustrate the need for long-term commitment to see positive change.

Summary:

Cover crops will use soil moisture relative to growth stage, biomass and environmental conditions. Cover crops can have a more cumulative effect on soil water the longer they are used in the operation. Seasonal differences will be evident with residue and cover crop water use; however, changes in the soil that promote better infiltration and water holding capacity can provide valuable benefits to the cash crop, especially in drier environments. It is important to monitor in-season water use of the cover crops and determine the proper termination timing to mitigate cash crop impact, while maintaining a focus on the long-term goal.

References:

Basche, A., and M. DeLonge. 2017. The impact of continuous living cover on soil hydrologic properties: A meta-analysis. Soil Sci. Soc. Am. J. 81:1179–1190. https://doi.org/10.2136/sssaj2017.03.0077

Basche, A., Kaspar, T. , Archontoulis, S., Jaynes, D., Sauer, T., Parkin, T., Miguez, F. 2016. Soil water improvements with the long-term use of a winter rye cover crop, Agricultural Water Management, Volume 172, 2016, Pages 40-50, ISSN 0378-3774, https://doi.org/10.1016/j.agwat.2016.04.006.

Chalise, Kopila Subedi; Singh, Shikha; Wegner, Brianna R.; Kumar, Sandeep; Pérez-Gutiérrez, Juan D.; Osborne, Shannon L.; Nleya, Thandiwe; Guzman, Jose; and Rohila, Jai S., "Cover Crops and Returning Residue Impact on Soil Organic Carbon, Bulk Density, Penetration Resistance, Water Retention, Infiltration, and Soybean Yield" (2018). Publications from USDA-ARS / UNL Faculty. 2139. https://digitalcommons.unl.edu/usdaarsfacpub/2139