Linda F. Benedict, Board, James E.
James E. Board, David Y. Lanclos, James Rabb and Bobby G. Harville
Planting soybeans after June 15 is a major production problem in Louisiana. For every day that planting is delayed after June 15, a soybean farmer can expect to lose an average of a half bushel of yield. Yet, farmers continue to do this for a variety of reasons. We have no accurate figures for how many farmers plant soybeans late. But we know the practice represents a substantial annual financial loss of tens of millions of dollars to our state.
The most common reason for late planting of soybeans is adverse weather. Weather conditions from May 1 to June 15, the optimal planting period for most varieties, may be too wet or too dry. Other reasons for late planting include double cropping after wheat, a delay in receiving a crop loan, a late-season rise in soybean prices or the farmer is preoccupied with other crops before attending to soybean planting.
Cultural recommendations for increasing late-planted soybean yield include reducing row spacing to 20 inches or less, increasing plant population above the level recommended for optimal planting dates and alleviating any environmental stress, such as from drought or water logging. Irrigation systems will alleviate drought stress. Grading fields to allow for drainage, eliminating uneven spots and adding ditches around fields can alleviate water logging.
Another way to increase yield at late planting dates is proper varietal selection. Because varietal rankings are not consistent between optimal and late planting dates, farmers cannot rely on information from the statewide soybean variety trials to determine what to plant. The objective of this research is to provide soybean producers with varietal recommendations specifically for late planting.
Our study involved 46 soybean varieties selected from the statewide trials and representing Maturity Groups V, VI and VII. Soybeans are classified into maturity groups based on the length of time needed to reach harvest maturity after planting. The higher the Roman numeral, the longer it takes for the soybean to reach harvest maturity. The varieties were planted at Baton Rouge on July 7, 1995, and July 2, 1996, in a randomized complete block experimental design with four replications. A companion variety study, consisting of 15 varieties selected from the 46 varieties grown at Baton Rouge, also was conducted in 1996 at the Red River Research Station at Bossier City in northwest Louisiana. We did this to determine if yield rankings between Baton Rouge and Bossier City were similar.
In addition, in both Baton Rouge studies, we collected data on plant dry weight at the start of seed fill and the length of the seed filling period. These additional data, along with plant height, were used in a regression equation for predicting yield. Our purpose was to compare actual and predicted yield to determine the accuracy of the regression equation. If accurate, the regression equation would provide us with a much faster and efficient method for identifying high-yielding varieties and lines at late planting dates. Such a tool would be useful in breeding programs aimed at developing varieties with improved yield at late plantings.
Averaged across 1995 and 1996, yields in the late-planted variety trial at Baton Rouge ranged from a low of 43 bushels per acre for TV5452 (Maturity Group V) to a high of 62 bushels per acre for Dyna-gro 3682 ( Maturity Group VII). Based on these results, a farmer could potentially increase yield by 44 percent by choosing the proper variety.
Using the guideline developed for the statewide variety trials (that recommended varieties are those yielding within 90 percent of the average yield of the top three varieties), 11 of the 46 varieties were recommended (Table 1). Most of the recommended varieties were from Maturity Group VII, supporting the recommendation that the best-yielding varieties for late planting are those with longer growing periods. Three Maturity Group VI varieties, however, also were recommended: P9692, HBK67 and H6686.
Yields in the Bossier City study ranged from 35 to 57 bushels per acre (Table 2). Surprisingly, the best-yielding variety in this study was A5885 (Maturity Group V). Yield rankings between Baton Rouge and Bossier City were not similar. Some of the highest-yielding varieties at Bossier City ( A5885 and DP3627) had modest yields at Baton Rouge, and some recommended varieties at Baton Rouge (Dyna-gro 3682 and NKS75-55) gave intermediate yields at Bossier City.
The general superiority of Maturity Group VII varieties at Baton Rouge did not occur at Bossier City. Although yield rankings from Baton Rouge can be extrapolated to other areas of southern Louisiana, these results indicate they cannot be extrapolated to northern Louisiana. Separate late-planted variety trials will have to be conducted in northern Louisiana to identify recommended varieties for that region.
Comparison of actual and predicted yields indicated that the regression equation, based on crop dry weight at the start of seed filling, plant height and length of the seed filling period, provided an accurate estimation of yield. In 70 percent of comparisons, predicted yields were within 6 percent of actual yields. The regression equation identified three of the four top-yielding varieties. Since the parameters entered into the regression equation are easy to obtain and can be determined in small one-row plots, the regression equation has potential for rapidly identifying high-yielding varieties or lines for lateplanted production. We will do more work with this regression model.
The authors express appreciation to the Louisiana Soybean and Grain Research and Promotion Board for providing funds to help support this research.
James E. Board, Professor, and David Y. Lanclos, former Graduate Research Assistant, both with the Department of Agronomy, LSU Agricultural Center, Baton Rouge, La.; James Rabb, Professor, Red River Research Station, Bossier City, La.; and Bobby G. Harville, Professor, Department of Agronomy, LSU Agricultural Center, Baton Rouge, La..
(This article was published in the winter 1999 issue of Louisiana Agriculture.)