Various indicators point to surface drainage problems negatively affecting soybean yields, especially in wet years. One common-sense indicator is to just view a young soybean field that has low spots following some big rains. The plants in the low locales are stunted, or maybe even dead. Data from irrigators in the southeast Missouri region provided to the annual Bootheel irrigation surveys also points in the same direction.
First, growers indicated in the surveys that soybean yields are 4 bushels to 5 bushels per acre higher in fields that have been laser-leveled. Second, yields are higher if the field was bedded up. Interestingly, the yield increase from both lasering and bedding is always greater on full-season soybeans than it is for double-crop soybeans.
I have always attributed that to the fact that full-season soybeans receive more rainfall than double-crop beans. According to research on flat- and bed-planted soybeans, both full-season and double-crop, there is an 8.8-bushel and 3.8-bushel yield increase for beds versus flat for full-season and double-crop soybeans, respectively. The data comes from 2003-08 and represents information from 142 fields.
To test the supposition that early-season rainfall plays a factor in increasing the benefits of beds, yield data from the Bootheel irrigation surveys were correlated to the amount of rainfall just following planting. Daily rainfall amounts were collected for the years 2003 to 2008 using the University of Missouri’s Delta Center weather station located at Portageville, Mo. Rainfall amounts were summed up for periods of two, three, four, five, six, and seven weeks following “typical” planting dates. May 7 and June 30 were used as the “typical” planting dates for full-season and double-crop soybeans, respectively.
The average flat-planted yield for each year was subtracted from the average bed-planted yield to obtain the difference in yield between the two bedding methods for both full- and double-crop beans for each year. This yield difference was then correlated to the rainfall amounts for each of the six periods of time after planting (i.e., two to seven weeks).
It was found that yield-rainfall amount correlations were better as the period of time became shorter. That is, the total amount of rainfall that occurred in the seven weeks after planting had little bearing in predicting the yield difference between flat and bedded systems. However, the amount of rainfall received in the two weeks following planting had good linear correlation.
If rainfall amounts during the first two weeks following planting exceed about 1.25 inches, yield begins to be lost if soybeans are flat-planted. For each additional inch of rain after the 1.25-inch point, about 5 or 6 bushels of yield is lost.
While bedding up beans sounds like a good recommendation on paper, it can pose some problems. Historically, the Bootheel irrigation survey shows that about 50 percent of full-season and 60 percent of double-cropped soybeans are flat-planted.
There are other concerns besides the cost of fuel for bedding up. Farmers who rotate rice and beans will be hard-pressed to bed up a bean field that will next year go back onto rice. Also, farmers with double-crop soybeans have only a small window of time to get their bean crop planted after the winter wheat has come off.
Another factor to consider is what exactly will be the amount of rainfall that will be received during the two weeks following planting. One is likely to experience smaller amounts of rainfall in the two-week period following double-crop soybean planting than would be encountered in the two weeks after full-season soybean planting due to the change in rainfall patterns.
In fact, from 1990 to 2008, 49 percent of the years had less than 1.25 inches for the two-week total on double crop soybeans. Full-season beans had 42 percent of its years less than 1.25 inches.
Putting soybeans on beds in the Mid-South appears to increase yield, which is most likely related to the amount of rainfall that is received after planting. The benefits of bedding increases as post-planting rainfall increases, thus full-season beans show higher yield increases to bedding than do double-crop soybeans. A farmer can use rainfall probabilities to help determine if bedding will work for him.