Dustin L. Harrell and Jason A. Bond

The growing season in southwest Louisiana is long enough to produce two rice crops. The second rice crop is known by several names including stubble and ratoon. The ratoon crop develops by regenerating tillers from buds on the stubble left behind after the first crop harvest. The costs associated with growing the ratoon crop are minimal compared with the first crop. Generally, these costs include nitrogen (N) fertilizer, irrigation, harvesting and grain drying. Additional inputs such as herbicides, insecticides and fungicides, which are used in the first crop, are not used in the ratoon crop.
 
In general, a rice farmer can expect to see ratoon yields of approximately one-third of that realized in the first rice crop. Although ratoon yields are much less than that of the first crop, there is a definite economical advantage of growing the ratoon crop in southwest Louisiana. Because of the importance of the ratoon crop, several research studies have been implemented over the past few years to help improve ratoon productivity. These studies have focused on identifying proper fertilizer nutrient timing and rates, as well as identifying stubble manipulation practices that may have the potential to increase ratoon yields.
 
Several stubble manipulation practices have the potential to increase ratoon rice yields. One such practice includes lowering the first crop harvest height to around 20 centimeters tall, which would be about half the typical height of 40 centimeters. Other practices include harvesting at the traditional harvest height followed by either flail-mowing or rolling the remaining stubble. Harvesting with a lower combine platform height, however, is the only stubble manipulation practice that could be implemented without purchasing additional equipment or without substantially increasing fuel costs associated with running equipment over the field a second time.
 
Ratoon stubble height trial

A two-year trial was initiated in 2006 to evaluate the effects of ratoon stubble height on ratoon rice growth, development and yield. The trials were conducted at the LSU AgCenter’s Rice Research Station South Farm in Crowley on a Crowley silt loam. Cocodrie and Trenasse rice varieties were used in the trials. The first rice crop was harvested with a small plot combine with the cutter bar adjusted to leave either a stubble height of 20 or 40 centimeters. After the main crop harvest, panicles in a flagged 3-meter linear section were tagged on a weekly basis as they emerged beginning at three weeks after harvest (3 WAH) and concluding 10 WAH.


Prior to whole plot ratoon harvest with the combine, the 3-meter sections were hand-harvested and panicle origins were determined to be either axial or basal. Basal panicles were defined as those panicles that developed from nodes at or below the soil surface and included any nodes less than 2 centimeters above the soil surface that had roots extending into the soil. Axial nodes were defined as those that originated above the soil surface and did not have any roots extending into the soil.
 
Ratoon yield results
 
In 2006, ratoon rice grain yield was not improved when the ratoon stubble was reduced to 20 centimeters. This can be explained partially by the narrow brown leaf spot (NBSP) outbreak experienced in 2006, which limited the yield potential of the ratoon crop. A fungicide was not applied in the trial in an effort to mimic the minimal chemical strategy used in commercial agriculture. In 2007, ratoon yields were not limited by disease, and both Cocodrie and Trenasse grain yields were improved when the initial stubble height was 20 centimeters (Table 1) compared with the traditional 40-centimeter stubble.
 
Ratoon heading and point of origin results
 
Differences in panicle point of origin on a cumulative weekly basis were observed for both rice varieties and initial stubble height in 2007 (Figure 1). Briefly, when the main-crop stubble was 20 centimeters, basal and axial panicle points of origin were similar 5 WAH for both varieties. A higher density of ratoon panicles originated from basal nodes at 6 WAH and beyond.
 
In contrast, when the main-crop stubble was 40 centimeters, approximately 75 percent of the emerged panicles originated from axial nodes at 5 WAH for both varieties. Densities of panicles emerging from basal and axial nodes were nearly identical 6 WAH. After 6 WAH, greater panicle density was observed from basal nodes. Therefore, the delay in maturity often associated with shorter main-crop stubble heights can be explained partially by the minimal exertion of panicles from tillers originating from axial nodes, which is more prevalent from the taller ratoon stubble associated with traditional harvest heights that leave 40-centimeter stubble.

Conclusions
 
In southwest Louisiana, rice ratooning is important because the total rice yield per area can be increased with minimal additional agricultural inputs. Data from this study suggest that harvesting the main crop at a lower-than-normal height, resulting in stubble of approximately 20 centimeters, will alter panicle point of origin in Cocodrie and Trenasse and, in some years, may increase ratoon yields.
 
The 2007 yield advantage of the 20- centimeter main-crop stubble height was associated with heavier panicles from tillers originating on basal nodes (data not shown). During years when a low ratoon yield potential exists, lowering the main-crop harvest height most likely will not improve ratoon yields. In all years, genetics, stubble health, availability of soil nutrients and water, and weather during ratooning precedes harvest height as the most limiting factor for ratoon yield potential. Producers need to evaluate ratoon potential before implementing a lower-than-traditional harvest height or applying post-harvest flail-mowing, which can increase production costs.
 
Acknowledgments
 
This research was funded in part by the Louisiana Rice Research Board. Special thanks to Richard Dunand, now retired, and research associates James Leonards, Ron Regan and Russell Dilly for their contributions to this research.

Dustin L. Harrell, Assistant Professor, Rice Research Station, Crowley, La., and Jason A. Bond, Assistant Professor, Delta Research and Extension Center, Mississippi State University, Stoneville, Miss., and formerly with the LSU AgCenter.

(This article was published in the winter 2009 issue of Louisiana Agriculture.)