Magdi Selim, Brenda Tubaña, Allen Arceneaux and Chris Coreil

Soil conservation production systems in agriculture are characterized by the presence of mulch residue left on the surface, which protects the soil from erosion. In sugarcane production there is considerable interest in the effect of the sugarcane residue, or mulch cover, on reducing soil sediment and nitrogen and phosphate nutrient losses. Numerous studies on several crops have shown that crop residue or surface mulch can enhance weed control and reduce off-target herbicide losses. This information is essential for the implementation of control measures or corrective actions needed to reduce chemical leaching and sediment losses from cropland and thus reduce contamination of surface water and groundwater. Some 10 years ago, the effectiveness of sugarcane mulch residue in south Louisiana on the retention of applied herbicides and their leaching losses in runoff was investigated at the LSU AgCenter. It was reported that significant amounts of applied herbicides were intercepted by the sugarcane mulch residue.

Since the mid-1990s, the Louisiana sugarcane industry has adopted a harvesting technology that involves use of a combine harvester, which cuts the cane stalks into short lengths, called billets. Extractor fans in the combine separate leaf material from billets and deposit the leaf residue on the soil surface. In Louisiana, a major economic concern is the effect of the presence of residue on sugarcane yield. A number of AgCenter studies reported reductions in subsequent sugar yields when the residue was not removed. Burning the residue prior to or following harvest reduces the effect of the residue on crop emergence in the spring and ultimately on sugar yield. A recent AgCenter study measured the long-term effect of post-harvest crop residue management on the sugarcane yield for three production cycles. Retaining the residue resulted in reduced yield and stalk populations. Burning resulted in an average sugar yield increase of 0.43 tons per acre over full retention of residue (no-till) and 0.28 tons per acre over sweeping, which is a method of mechanically removing the residue with sweeper brushes. The researchers concluded that yield reductions from not removing crop residue were a result of timing and confined to ratoon crops within a production cycle.

Lower sugar yields when the residue was mechanically removed by sweeping were likely due to damage caused by the mechanical actions of the sweeper brushes. For other grass crops, such as perennial ryegrass and tall fescue, mechanical removal of residue has been reported to be as effective as burning. Therefore, sugarcane yield reduction can be minimized if residue is mechanically removed from the top of the cane rows. Subsequently, improvement in mechanical residue removal would result in minimizing stubble damage as well as soil losses from the top of the sugarcane rows. An improved sweeper may be key to sugarcane producers adopting sweeping as an alternative to burning.

LSU AgCenter researchers conducted a study focused on implementing residue management strategies by using a new sweeper capable of removing sugarcane residue off the top of the rows with minimal soil surface losses and damage to stubble cane. To achieve this goal, three management strategies were evaluated: (1) burning the mulch after harvest; (2) sweeping the mulch off the top of the row; and (3) leaving the mulch undisturbed. Sugarcane plant populations and cane and sugar yields were measured for each treatment. Runoff losses were measured along with water quality parameters including total and dissolved sediment, turbidity, nitrate and phosphate.

The focus here was to evaluate a mechanical residue removal implement for minimizing soil and nutrients losses and any reduction in sugarcane yield. The implement was used with the assumption that improving mechanical residue removal is prerequisite for minimizing stubble damage as well as soil losses from the top of the sugarcane rows.

Field studies were carried out at four experimental sites during the 2012 through 2016 growing seasons. In 2012, two experiments were established near Paincourtville, one at Dugas Farm and one at Gravois Farm. At Gravois, three plots larger than one acre each were selected, and the following management practices were implemented: (1) burning the mulch after harvest; (2) sweeping the mulch off the top of the row; and (3) no-till, or leaving the mulch on the soil surface. The researchers measured water quality through edge-of-field sampling. The experiments at Dugas Farm consisted of three treatments and two replications. Each plot consisted of six rows 300 feet long. For both Gravois and Dugas sites, all treatments were applied in January 2013 after the plant cane was harvested in December 2012. Stalk counts were collected in August 2013, and the first stubble cane was harvested in December 2013 for both sites. Ten stalk samples were collected at harvest and processed at the LSU AgCenter Sugar Research Station in St. Gabriel for sucrose analyses. In the meantime, water quality was monitored throughout the growing season, and samples were collected when rainfall amounts were sufficient to initiate runoff. Collected samples were analyzed for sediment, nitrogen and phosphorus.

During 2015, a site at Duson was initiated and consisted of the same three treatments and two replications. The plots were three rows wide and 480 feet long. Second stubble was harvested in December 2014. A site at the Sugar Research Station was also initiated in 2015 and consisted of the same three treatments with four replications. The first stubble cane was harvested in November 2015. This experiment was continued in 2016. The treatments were applied in January 2016. The second stubble cane was harvested in November 2016, and stalk counts were collected.

Sugarcane Yield

For first stubble cane, the use of the modified sweeper was similar to the conventional burn treatment. This result was consistent for the two locations for the 2013 growing season. For the Gravois site, the total yields were 34.8 tons per acre for the sweep treatment and 35.1 tons per acre for the burn treatment. For the Dugas site, yields were 33.3 tons per acre for sweeping and 33.8 tons per acre for burning. These results contrast with earlier work reported on sugarcane since 2001 when yield losses ranged from 9 to 14 percent when sweepers were used compared with conventional burning. In 2014, at the Gravois site, the total yield was 28.7 tons per acre for the sweep treatment and 28 tons per acre for the burn treatment, a difference of only 2.5 percent.

For 2015, yield results from the Duson site were relatively low due to extreme drought conditions in the surrounding region, where less than 2 inches of rain were recorded between June and harvest on Oct. 2. Such droughty conditions were responsible for the observed higher yield when the mulch was not removed due conservation of soil moisture under no-till. Nevertheless, cane yield and total sugar were comparable for the sweeper and for the burn treatment. Therefore, in spite of summer drought at Duson, the sweep treatment did not result in yield reduction when compared to the conventional burn treatment. The St. Gabriel site did not experience droughts of the magnitude of Duson during 2015, and considerably higher yields were realized. In fact, cane yields were comparable for all treatments; 30.8 tons per acre for burning, 32 tons per acre for mulch and 30 tons per acre for sweeping.

Based on results from the Duson and St. Gabriel sites during 2015 and 2016, yield and sugar were comparable for the sweep and the burn treatments. Average yields for different locations and varieties are given for all treatments, showing evidence that using a modified sweeper is recommended as a best management strategy. This finding was based on results from four growing season, four varieties and four sites.

Water Quality

Effluent or runoff water samples for 2013 and 2014 for all management treatments were collected from the Gravois site following each rainfall that triggered runoff. Collected runoff samples were analyzed based on U.S. Environmental Protection Agency protocols that included total and suspended solids, turbidity, nitrate, ammonium and dissolved phosphate.

During 2013, 22 water quality samples were collected compared to only 14 samples in 2014. During the sampling period, rainfall amounts were significantly lower in 2013 (28.7 inches) than in 2014 (35.2 inches). Sampling was initiated following last herbicide application and cultivation commonly carried out in early May and prior to the closure of plant canopy.

During 2013 and 2014, total solids in the effluent from all treatments did not exhibit consistent differences. Results from suspended solids mimic closely those for total solids. Increased turbidity was observed during 2014 compared to 2013, which is likely due to higher rainfall in 2014. Concentrations of phosphate and nitrogen in the effluent from the runoff were extremely low and did not exceed 1 part per million (ppm) during the entire sampling period with a mean value of 0.4 ppm for all treatments. In earlier studies, phosphate did not exceed 1.3 ppm in the effluent over two growing seasons (2008-2009). For nitrogen, nitrate concentrations were extremely low and exhibited no obvious pattern over the sampling period. The only exception is for the burn treatment, where significantly higher nitrate levels were observed. This high concentration was perhaps due to burning the residue. Nevertheless, nitrate levels after mid-July were extremely low for all treatments (0.2 to 1.1 ppm).

These results imply that the environmental effect of nutrient losses from sugarcane fields were minimal for all management strategies investigated in the study. Specifically, the use of a sweeper as a management practice did not contribute additional nutrient losses from sugarcane fields when compared to conventional burning. Water quality data — for example, sediment losses of phosphate and nitrogen from the edge of a field — are comparable for sweep, conventional burn and the no-till treatments.

The findings from this study were:

• There was no significant difference in dissolved solids and total solids, turbidity, phosphorus and nitrogen. In fact, the influence of the sweeper on soil and nutrient losses was comparable to runoff from burning and mulch management strategies.

• Sugarcane yield was not significantly different between mulch or burn treatments. This finding was based on results from four growing seasons (2013-2016) and four locations.

Magdi Selim holds the A. George and Mildred G. Caldwell Endowed Professorship in Agronomy and Soils, Brenda Tubaña holds the Jack E. and Henrietta Jones Professorship, and Allen Arceneaux is a research associate in the School of Plant, Environmental and Soil Sciences. Chris Coreil is a conservation agronomist with the Natural Resources Conservation Service in Alexandria, Louisiana.

(This article appears in the fall 2017 issue of Louisiana Agriculture.)

The sweeper attachment mechanically removes residue with "brushes." Photo by Allen Arceneaux

A closer look at the attachment that mechanically removes sugarcane residue from a field. Photo by Allen Arceneaux