How much will tile drainage increase soybean yields?
Year to year it depends, of course, on how wet your soils are early in the growing season. But a study of 20 years of data from both experimental trail and producer fields in the North Central United States shows a 4-8% yield advantage for artificial subsurface drainage.
The 4% figure — 2.3 bushels per acre — is the average from producer fields.
The 8% figure — 4.1 bushels per acre — is from experimental fields.
The yield advantage in producer fields can be attributed earlier planting that is possible when artificial drainage is in place, says Hans Kendall, North Dakota State University Extension agronomist. Kendall was involved in the study conducted by the North Central Soybean Research Program (NCSRP), which is made up of universities, Extension services and soybean commodities groups.
“Across 20 tile drainage trials in North Dakota from 2011 through 2019 the average soybean yield increase was 7%, including drier years,” Kendall says. “The take-home message is that across years tile drainage provides higher soybean yields.”
Field trial yields
The yield and management data from experimental fields came from replicated field experiments conducted in Iowa, Missouri, Minnesota and North Dakota over the past 20 years. All experiments were rainfed. They did not receive supplemental irrigation. Each trial included a side-by-side comparison of artificial drainage versus naturally drained treatments.
Daily precipitation data were retrieved from meteorological stations located less than 18 miles from the experimental sites. The amount of precipitation received in the 30 days before planting was tallied for each site-year. The 30-day total was considered a reasonable indicator of excess water in the soil and workability of the field early in the season.
Data on producer yield and management practices were collected from more than 2,800 fields over four years (2014-2017) in Iowa, Illinois, Indiana, Kansas, Michigan, Minnesota, North Dakota, Nebraska, Ohio and Wisconsin. Soybean producers indicated type of drainage for each field by selecting among three options: natural drainage, surface artificial drainage, and subsurface artificial drainage.
Producer fields were grouped based on similarity of climate and soil. Weather data for each surveyed field were obtained from Daymet, a surface weather data source run by NASA. The amount of precipitation each field received during the 30-day period before planting was calculated for each field. The researchers focused their assessment on dryland fields.
While there was a 4.1 bushel per ace advantage for artificial drainage in the experimental fields, there was still variation in the magnitude of the yield difference, ranging from negative 1.7 to 11.5 bushels per acre across site-years. In only 6% of the cases, yield difference between artificial drained and the naturally drained experimental fields was zero or negative.
While the average producer yield was 2.1 bushels per acre higher for subsurface artificial drainage versus naturally drained fields, there was variation in the magnitude of the artificially drained and naturally drained yield difference, ranging from negative 1.7 to 5.9 bushels per acre. In 10% of the cases, the yield difference between artificially drained and naturally drained fields zero or negative.
Surface drainage positive
In North Dakota, the average yield from surface drained fields was 1.4 bushels per acre higher than from non-drained fields, but the difference didn’t meet the study’s probability value of 0.092 to be classified as statistically significant. The difference between artificially surface drained and naturally surface drained ranged from negative 1.7 to 5.9 bushels per acre.
The reason the average yield difference was not considered significant is “partly due to the confounding factor that those who use surface drainage are in areas with water issues and those who do not use surface drainage probably did not use it and the research team could not separate out the reason why farmers did not use surface drainage,” Kendall wrote in an email.
Despite the confounding factors, the artificial surface drainage did have an effect, he said.
“The word ‘statistically’ is a term used by scientists and is based on the standard of the paper,” Kendall said. “If researchers needed to be 95% sure, the probability value would equal to or less than 0.05. The probability value in this experiment was 0.092 so the researchers are 91% sure that the differences are real. In many of our variety trial results we use a probability value of 0.10 or we are 90% sure differences between varieties are different. So in extension and farmers looking at data, I would say that being 91% sure that the observed difference is real is more realistic than saying there is no statistical difference.”
The conclusion, Kendall wrote, “is that tile drainage has a larger effect than surface drainage and the best management would combine both surface and subsurface drainage when needed.”
See the complete report online.