Another Case for Long-Term Agricultural Research

In a previous blog article, information was presented from research that indicated that the benefits of switching from a conventional tillage system to a no-till system will only be realized over the long-term. The results suggested that research designed to evaluate those benefits in any environment should be conducted for 15 years or longer.

Since research with cover crops is usually designed to determine how they may enhance/benefit soil health, it seems logical that this practice should also be evaluated over the long-term. In other words, long-term vs. short-term research with cover crops should be the goal. The only likely exception to this philosophy will be research that assesses using high-biomass cover crops to aid in the control of problematic weeds–this can likely be accomplished with short-term research.

An article titled “Impacts of cover crop planting dates on soils after four years” (Agron. J. 2020:112:1649-1665) authored by Ruis et al. provides results that support the view that cover crops research should be long-term. A summary of the conduct of and results from this research follow.

•   The premises of the study were that 1) cover crop planting date may critically influence cover crops’ subsequent effect on soil properties–i.e., the presumed reduced growth resulting from delayed planting of the cover crop in the fall may alter cover crops’ effect, and 2) cover crops that contain mixes of species are thought to have a greater positive effect on soil properties.

•   Research was conducted for four years at three no-till continuous corn and corn-soybean sites (six site-cropping system combinations each year) in south central and eastern Nebraska to evaluate how fall planting date (pre- and post-harvest of corn/soybean crops) and cover crop type (cereal rye, or cover crop mix of winter pea, hairy vetch, cereal rye, and radish, or no cover crop) affected cover crop productivity and the subsequent effects on soil physical properties, organic matter (OM), and cover crop biomass carbon input.

•   Post-harvest cover crop plantings were generally made 5-9 weeks after pre-harvest plantings in both corn and soybean crops. Pre-harvest cover crops were broadcast-planted, and post-harvest cover crops were drill-seeded.

•   Termination of cover crops before corn planting occurred in mid- to late-April, and termination before soybean planting occurred in late-April through early May.

•   Cover crop biomass was determined each spring at termination of the cover crop.

•   Measurements were taken and soil samples were collected from continuous corn plots and from the corn plots in the corn-soybean rotation at the end of the 4 years. Soil penetration resistance, water infiltration rate into soil, soil bulk density, OM concentrations in soil, soil organic carbon (C), and total nitrogen (N) in soil were determined.

•   The cover crop mix was dominated by the cereal rye (about 90% rye) in this study.

•   Over the 4 years, cover crop biomass production averaged 730 and 513 lb./acre from the pre- and post-harvest plantings, respectively. The difference was significant in four of the six site-cropping systems, but the greatest biomass production was only 1330 lb./acre. Thus, cover crop biomass production was low.

•   Over the 4 years, the rye cover crop produced an average of 742 lb./acre biomass per acre, which was greater than the average 504 lb./acre biomass from the mixed cover crop. The difference was significant in only two of the six site-cropping systems. Again, cover crop biomass production was low regardless of cover crop type.

•   Across the 4 years, cover crop planting time and cover crop type had no consistent effect on cover crop biomass C input to soil. Where differences were significant, pre-harvest planting and the rye cover crop resulted in the greatest biomass C input to soil.

•   After 4 years, cover crop planting time and cover crop type had no significant effect on either water infiltration rate into soil or soil organic C.

•   After 4 years, cover crop planting time and cover crop type rarely had a significant effect on either soil penetration resistance or soil bulk density.

•   Cover crop type had no significant effect on total soil N.

•   Cover crop type effects on soil properties did not generally vary between the continuous corn and corn-soybean cropping systems.

•   Cover crop biomass production was not correlated with soil properties at any site or across sites.

•   The authors concluded from their extensive literature review coupled with their results that 1) cover crop biomass production must be sufficiently large–likely >2500 lb./acre–to effect detectable changes in soil properties, 2) the short-term nature of this project was a likely contributing factor to the few differences in soil properties measured in this study, and 3) the additional seed cost of a cover crop mix may not be warranted if a cereal rye component that will likely dominate is included.

•   Overall, results from this research show that: 1) the limited effects from cover crops on soil properties in this study with consistently low biomass production highlight the role of biomass production in effecting changes in those soil properties when cover crops are included; and 2) pre- vs. post-harvest planting of cover crops and using cover crop mixes vs. only cereal rye may have only limited effect on soil health enhancement, especially in the short-term. Thus, it should not be assumed that these two practices, when used in the short-term, will necessarily enhance the positive effects from using cover crops.

So, to iterate the content of the second paragraph of this article, and to mimic the conclusion in the previous blog article mentioned in the first paragraph above, the following is offered.

These results indicate that cover crops research, like tillage research, should be conducted for an extended period to ensure that the hoped-for, long-term soil benefits from their inclusion in a production system are objectively and accurately determined.

As stated above, the data in the above-cited article were obtained in Nebraska. Thus, there is no assumption that the absolute values from that research will be the same as those obtained from similar studies that may be conducted in the Midsouth. However, it is quite likely that the cited findings on duration of time (short- vs. long-term) needed to detect the positive benefits of cover crops adoption will be representative of those from Midsouth studies–i.e., there is little doubt that short- vs. long-term cover crops vs. no cover crops studies can or will provide misleading results no matter where the study is conducted. This is a point that funding entities–both public and private–should consider when determining whether or not to fund cover crops research.

Composed by Larry G. Heatherly, June 2020, larryheatherly@bellsouth.net