Linda Benedict, Haggard, Beatrix J, Lofton, Josh
Josh Lofton and Beatrix Haggard
Wheat-soybean double-crop rotation is important in Louisiana, with more than 85 percent of the total wheat acreage planted in a double-crop soybean system. This system attempts to use climate, land and other production resources to optimally produce both wheat and soybeans on the same field in the same year. This entails planting soybeans almost immediately following wheat harvest around late May or early June. Wheat residue can create many challenges in obtaining an acceptable soybean stand; therefore, various residue management techniques need to be evaluated.
While numerous research studies have shown the benefits of many wheat residue management practices on soybean production, few have discussed the pronounced effects these techniques could have on the quality and natural fertility of the soil. These quality characteristics can include plant-available nutrients, soil pH, organic matter, and short- and long-term carbon content – all of which affect long-term productivity and sustainability.
These soil-quality characteristics have become a topic of increasing interest in recent years, not only to researchers but also to landowners and managers. This is due to the effects of soil quality parameters on the long-term sustainability of many production systems as well as their effects on environmental concerns, such as carbon management and storage.
Depending on the management techniques used, soil can either become a site of carbon storage or a source of carbon dioxide in the atmosphere. Therefore, LSU AgCenter scientists are studying the effects of various commonly implemented residue-management techniques and their implications on soybean production and key soil quality characteristics.
Field trials were conducted in the 2011-2012 growing season at the Northeast Research Station near St. Joseph, La. Following winter wheat harvest, four wheat residue management techniques were implemented: leaving wheat residue on the soil surface, burning the residue, mowing the residue and tilling in the residue. The first three residue-management techniques were conducted in a no-tillage system.
At soybean maturity, plots were harvested and a sample weight was used to determine soybean yield. Soil samples were collected during early soybean growth stages and at the end of the season. They were used to measure soil pH, organic matter and soil carbon content among other soil-quality characteristics.
Management techniques significantly affected soybean yield (Figure 1). When wheat residue was minimized by mowing or removed through tillage or by burning, soybean yield increased substantially. If the only concern was which management practice optimized soybean yield, any of these options would be acceptable. However, soil quality is equally or more important to the long-term sustainability of Louisiana agriculture. While there are many ways to evaluate soil quality, values that have short-term variability but can indicate long-term trends are total carbon and nitrogen.
When looking at the surface soil (0-3 inches), tilling or burning residue greatly affected carbon levels compared to the treatments where residue remained or was mowed (Figure 2). A similar trend was seen with nitrogen; however, increased residue management resulted in a more linear decrease, with the greatest differences between treatments where residue remained and was burned.
While less pronounced, the sub-surface soil (3-6 inches) showed a trend similar to surface soil from both carbon and nitrogen levels among different residue-management techniques (Figure 2). This less-prominent effect is due to deceased carbon and nitrogen in these lower levels.
Additionally, while a decreased carbon was found with tilled treatments in the surface soils, percent carbon levels for the sub-surface levels were similar to treatments when residue remained and was mowed (Figure 2). These increased or similar values for the surface and sub-surface levels resulted from mixing surface and sub-surface soils by tilling.
Overall, burning and tilling the wheat residue provide a cleaner seedbed, which allowed for optimum germination and stand establishment of the double-crop soybeans. However, these practices are not long-term sustainable solutions because of the intensive depletion of soil carbon and nitrogen. This makes the decision on how to manage wheat residue in a double- crop system difficult for Louisiana land managers.
Implementing residue management techniques that provide high soybean yields and maintain or increase soil quality parameters, such as mowing of wheat residue, can ensure double-crop soybeans are both productive and sustainable long term.
Josh Lofton is an assistant professor at the Macon Ridge Research Station in Winnsboro, and Beatrix Haggard is a soils specialist, Northeast Region in Winnsboro.
(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)