Submitted to: Open Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2015
Publication Date: 10/26/2015
Citation: Lascano, R.J., Krieg, D., Baker, J.T., Goebel, T.S., Gitz, D.C. 2015. Planting Cotton in a Crop Residue in a Semiarid Climate: Water Balance and Lint Yield. Open Journal of Soil Science. Open Journal of Soil Science, 2015, 5, 236-249.
Interpretive Summary: On first analysis the idea of planting cotton into a crop residue is a good idea and has several advantages. For example, in the Texas High Plains a common practice is to plant cotton into a wheat residue that provides a mechanical barrier to the cotton seedling and protects it from wind damage. Also, the wheat residue provides a barrier to slow down soil water evaporation. However, the problem is that there is a cost that seldom is taken into consideration and that is that it takes water to grow the residue. Further, we have determined that the wheat residue may actually accelerate the loss of water due to evaporation by the so-called wicking effect. The wheat residue is normally terminated by applying herbicide and as the wheat dies it provides a capillary system that accelerates the transport of water from the soil to the atmosphere. Part of the problem is that it is difficult to measure the wicking effect under field conditions as the daily rates are of the order of less than 1mm of water per day (0.04 inches per day). The field instrumentation that we have to measure soil water content does not have the sensitivity to measure soil water to this accuracy. Nevertheless, this daily loss has to be put in perspective and a loss of this rate over a period of 20 days represents 20 mm of water (0.8 inches) and could be the difference to be able to germinate and emerge the cotton crop planted into the residue. This means that the water that is used to grow the residue will subsequently prevent the cotton crop that is planted into the residue to be established. Thus, the conservation practice of planting cotton into a wheat residue is not recommended in areas where water is limiting such as the Texas High Plains, particularly during the spring-time, and where irrigation water is limited. At the end the question is one of tradeoff, i.e., should the water be used to grow a wheat residue that has definite advantages to cotton seedlings in the spring time, or should the water be left in the soil so that it may be used to germinate and emerge the subsequent cotton crop planted. We did field experiments to determine the feasibility of using this practice of planting cotton into a terminated wheat residue and we determined that at least for the two growing seasons and for the two sites used in our trials, there were no advantages and thus we cannot recommend this agronomic practice. Are there any advantages? The answer is yes. The residue will over a 3-5 year time period will increase the soil organic matter content, improve soil structure that is more conducive to increase infiltration of water, increase soil organic carbon, increase the soil’s water holding capacity and thus storage. These factors over a long-term are well know to increase crop yields and are practices that if the water is available are viable and recommended.
Technical Abstract: Cotton (Gossypium hirsutum L.) is planted on more land area than any other crop on the Texas High Plains. Much of this area is considered highly erodible and requires a conservation compliance program to participate in government farm programs. Because this region is semiarid and because irrigation water is increasingly limited, water conservation and efficient use of water are necessary to maximize cotton lint yields. One popular conservation compliance practice used is to plant cotton into a chemically terminated small grain crop, i.e., residue that provides wind protection to the cotton seedlings. We hypothesized that in a semiarid region the use of a small grain cover crop under irrigated conditions would use more water than it conserves compared to conventional tilled cotton, thus reducing cotton lint yields. To test our hypothesis we conducted separate field studies over two growing seasons and on two soil textures, a loamy fine sand and a clay loam. The main treatments were tillage systems (conventional and conservation using terminated wheat residue). The two split plot treatments were water supply based on replacement of calculated grass reference evapotranspiration (ETo). Tillage did not affect the amount of water used by the cotton crop at either location (< 7% difference, P > 0.05) except for an 80% ETo irrigation treatment at a single location where the bare soil treatment used 10% more water than the residue treatments for both years. The residue treatment decreased (P < 0.05) cotton lint yields at both locations by 12% except for the 50% ETo single irrigation treatment in which the residue treatment yielded 14% more lint than the bare soil treatment. The use of terminated wheat residue had no impact on soil water storage during any part of the year. During a 5-month period associated with wheat growth, the wheat evapotranspiration was 20 to 40 mm more water (P < 0.05) than that lost through soil water evaporation from the conventional treatments. The use of terminated wheat residue did not benefit the water balance of the cotton crop, and was associated with decreased cotton lint yields. The results were consistent with our working hypothesis, and disproved the idea that planting cotton into wheat stubble cover increases water use efficiency.