Eddie P. Millhollon
The rich, fertile soils of the Red River valley of northwestern Louisiana have supported cotton production for decades. Unfortunately, as in most agricultural soils, continuous cultivation has resulted in a steady decline in native soil fertility, especially organic matter. Although organic matter makes up only a small percentage of the total soil weight, it has a significant influence on chemical and physical characteristics. Organic matter is the major source of phosphorus, sulfur and nitrogen in the soil and can hold up to 20 times its weight in water, contributing greatly to soil water-holding capacity and water infiltration rates. It is therefore understandable why the steady decline of such a valuable natural resource in continuously cultivated soils raised the concerns of David Melville, a Louisiana Agricultural Experiment Station researcher. More than 40 years ago, he began a long-term study to see if winter cover crops could possibly arrest or reverse the decline in soil fertility, thereby maintaining cotton yields.
This study is located on the LSU AgCenter Red River Research Station in Bossier City, La. Since 1959, eight treatments have been studied for effects on soil fertility and cotton production (Table 1). Over the years, some of these treatments have required minor changes because of availability of cover crop seed or poor cover crop performance. Treatment plots consist of six 40-inch rows 210 feet long arranged in a randomized complete block design with four replications. Cover crops are planted each fall after cotton is harvested and the remaining stalks are shredded. In mid-April of each year, cover crops are clipped using a flail cutter, then disked under. All nitrogen fertilizer is applied either pre-plant to cotton or as a sidedressing after planting. Cotton is planted approximately 10 days after incorporating the cover crops. The use of herbicides, cultivation and insecticides are the same for all plots. Cotton yields are obtained by harvesting the center rows of each plot with a two-row spindle picker. In most years, a second harvest is conducted.
Figures 1 and 2 show how cotton yields have been affected by the different cover crop treatments from 1994 to 1998 and over the entire 40-year duration of this study. Over the five-year period (1994-1998) summarized in this report, cotton following hairy vetch with no supplemental fertilizer has produced the highest yields (Figure 1). This trend has been consistent over the 40 years of this study (Figure 2). The two treatments that have been the best for cotton production are hairy vetch grown as a winter cover crop followed by no additional nitrogen fertilizer or supplemented with 40 pounds of nitrogen per acre at the time cotton is planted.
A winter cover of hairy vetch has resulted in the highest levels of organic matter in the upper 6 inches of soil compared with all other treatments. The 0.61 percent organic matter content in the hairy vetch plots is nearly double that measured in 1976 (0.35 percent). It is also of interest to note that the organic matter of all treatments that included a winter cover crop had higher organic matter levels than treatments that remained fallow in winter.
In summary, this long-term study continues to demonstrate the benefits of alternating a summer cotton crop with a winter cover crop, especially hairy vetch. A winter cover crop not only results in superior cotton yields, but it also supplies residue for organic matter maintenance, thus increasing the water-holding capacity and nitrogen availability of the soil.
Eddie P. Millhollon, Associate Professor, Red River Research Station, Bossier City, La.
(This article was published in the winter 2002 issue of Louisiana Agriculture.)