Mahala Gambill, Shirley, Mark G.
Mahala "Haley" Gambill and Mark Shirley
Crawfish aquaculture is the most profitable aquaculture endeavor in Louisiana, representing roughly 69% of the total gross farm value generated across all commercial aquaculture enterprises in 2022, according to the Louisiana Summary Agriculture and Natural Resources 2022. Crawfish producers in south Louisiana create a suitable wetland habitat for crawfish to survive, reproduce and grow to harvest size in shallow water impoundments used for both crawfish and rice.
The red swamp crawfish (Procambarus clarkii) and the white river crawfish (Procambarus zonangulus) are the species of greatest commercial importance in Louisiana. Crawfish receive their nutrition from sources within the pond, based largely on a detrital food chain. Put simply, a forage crop serves as the fuel for a food web, with crawfish at the top of the food web. Therefore, establishment of a forage crop that adequately provides material to the detrital pool throughout the season is an important aspect of food resource management.
In fields where a crawfish crop follows a rice crop, it is the stubble, harvest debris (straw) and ratoon rice growth (the production of a second rice crop in one cropping season) that create the food web for the duration of the crawfish season. Rice is typically planted in March or April for harvest in July or August.
Ponds are drained during summer to establish a forage crop that serves as the foundation of the detritus-based food web when the pond is reflooded before crawfish harvest begins. Management of the ratoon rice crop differs from that of the first rice crop because chemical inputs, such as herbicides, insecticides and fungicides, are seldom used. Ratoon-specific practices include bushhogging the stubble; rolling the main-crop stubble following harvest with an implement that incorporates it into the soil; or spreading of the straw to prevent clumping and the reduction of regrowth in the combine row.
Additionally, delayed plant deterioration and ratoon regrowth may minimize dissolved oxygen demand in a crawfish pond during the fall because much of the biomass is in the form of green, living plant material, rather than decomposing straw and stubble. This also leads to staggered forage availability, with the forage components entering the detrital pool at different times during the production season. Previous ratoon stubble research has focused primarily on grain yield, yet few studies have assessed methods of ratoon crop management for the food supply of red swamp crawfish produced in rice ponds.
Four different stubble management schemes were evaluated at the LSU AgCenter H. Rouse Caffey Rice Research Station in Crowley during the 2021-22 crawfish season to identify the effects of ratoon crop manipulation methods on forage supply for crawfish. The treatments included bushhogging the stubble, rolling the stubble and burning the straw off the top of the stubble. A control treatment involved scattering the straw and not manipulating the stubble in any way.
In addition, this research aimed to identify the effect of fertilization on crawfish food supply. Fertilizer was added to one set of treatments and not to the other set. Typically, 90 pounds of nitrogen (N) per acre is recommended for ratoon rice production regardless of if stubble management is used or not. Total biomass was measured in each treatment monthly from October to May. Biomass evaluations were made on vegetative (leaves and stem) dry weight. Dry weight determinations were made after drying tissue in a forced-air oven at 70 degrees Celsius for 48 to 72 hours. An effects parameterization analysis of variance (ANOVA) and a multiple comparison of means were performed to assess the effect of the four common postharvest management methods on crawfish food supply during the season. A two-sample T-test was used to assess the difference in biomass between fertilized and nonfertilized stubble.
Overall, there was no significant difference between the stubble treatments. The biomass weights from bushhogging and rolling do not indicate a significant statistical difference to denote that their use is more beneficial, especially when factoring in the extra required time and fuel costs associated with them. Although not significant, the control group and use of rolling show more biomass throughout the season. Rolling of the stubble showed a persistence of biomass lasting longer throughout the season. The only significant difference in this study was due to the addition of fertilizer after the rice crop was harvested.
Management of the forage crop for crawfish under a rotational strategy is principally related to ensuring proper regrowth from the ratoon crop and minimizing the negative effects on water quality from breakdown of straw and debris from the grain harvest. Other studies note that rice stubble responds best to applications of fertilizer, followed by irrigation, shortly after grain harvest, as extended delays in supplying moisture to the stubble during dry conditions can result in poor regrowth.
The results of this study suggest that producers should evaluate the high operational costs, including those of time and resources, in methods of bushhogging and rolling of stubble to and consider instead the benefits of the application of fertilizer to rice stubble for seasonlong food supply for crawfish production.
Mahala "Haley" Gambill is an area agent in Terrebonne and Lafourche parishes for fisheries and coastal issues for the LSU AgCenter and Louisiana Sea Grant. Mark Shirley is a marine agent for the LSU AgCenter and Louisiana Sea Grant.
This article appears in the summer 2024 issue of Louisiana Agriculture.