Xueyan Sha and Steven D. Linscombe

About three quarters of Louisiana rice is grown in the southwestern region of the state. In recent years, ratooning (a second harvest from the same planting) has become a common practice for many rice growers in this region. Ratooning allows Louisiana rice growers to compete with rice growers in other states where the environment is more favorable for rice growing.

Because of the environmental limitations that restrict the time when rice stands can be successfully established or panicles can flower and fill grain optimally, rice in Southwest Louisiana is planted between late February and early July. Recommended planting dates for this region range from March 15 to April 20. To have a successful ratoon crop, rice must be planted before mid-April. Rice planted before the window of optimum dates usually has slow germination and emergence, poor stand establishment, increased production costs, and reduced grain production. Planting rice after the optimum dates can result in higher pest incidence and shorter day length (less photosynthesis) during grain filling, resulting in low yields. Because the current planting date recommendations are based on studies with relatively older varieties, there is a possibility that the pattern of response to planting date of recent or future releases may be different.

Milling quality, especially head rice yield (percentage of whole milled kernels), is equally as important as grain yield. Milling yield determines the unit ($/100 pounds) economic value of the crop. Research conducted in other states suggests that planting dates might also affect rice milling quality.

Studies have been conducted continuously at the LSU AgCenter Rice Research Station at Crowley during the past decade to evaluate the effects of planting dates on main crop yield, ratoon yield, milling quality, and agronomic performance of major rice varieties and experimental lines. Information generated from these studies is important not only in evaluating the stability of rice genotypes under the various environmental conditions posed by different times of planting but also in determining the optimum planting dates for different rice varieties.

Field data were obtained from experiments conducted from 2001-2004 at the Rice Research Station. Each year, between 12 and 15 rice varieties and advanced breeding lines were tested. Tests were planted every two weeks from late February to early July. All tests were no-till water-seeded at a seeding rate of 150 pounds per acre. At maturity, a small sample was hand-harvested from each plot for milling yield determination. For tests seeded on or before April 15, plots were fertilized and flooded approximately 4 to 6 days after harvest of the main crop to evaluate ratoon crop potential.

A broad range of genotypes were evaluated, including conventional long-, medium- and short-grain varieties, Clearfield (herbicide-tolerant) varieties, and hybrids. Rice genotypes in this study were new releases by the rice research stations in Louisiana, Texas and Arkansas, and experimental lines from Louisiana, as well as hybrids developed by RiceTec, which is a Texas-based private company and the only program that develops and commercializes rice hybrids in the United States. Entries were grouped into conventional long-grain, conventional Clearfield, conventional medium/short-grain, and hybrids:

• Long-grain: Cypress, Cocodrie, Cheniere, Trenasse, Saber, Ahrent, Wells, Francis, LA2051, LL184, and LL355 
• Clearfield: CL121, CL131, CL141, CL161, and CL029
• Medium/Short-grain: Bengal, Earl, Jupiter, Pirogue, LA2028, LL001, and LL401
• Hybrids: XL8, XL710, and CLXL8   

For the main crop overall, rice seeded in mid-April produced the highest grain yield (Figure 1). Grain yield increased as planting date was moved forward from February to mid-April then declined gradually with planting through early and mid-May. However, grain yield declined dramatically when seeded after mid-May.

Differences in response to planting date were noted between genotypes (Figure 1). Conventional long-grain and Clearfield types generally followed the trends noted above. Conventional long-grain and Clearfield types responded somewhat differently to planting date than medium-grain types. Medium-grain types were generally lower yielding when planted between the end of February and the middle of April and were generally higher yielding when planted after the middle of April. Medium-grain types had relatively constant yields when planted between the middle of April and the middle of May. Long-grain types showed a general decline in yield during this same period. Hybrids had the highest yield, regardless of planting date. Peak yields were associated with planting around the first of May, which was about two weeks later than the optimum planting date for peak yields with the conventional long- and medium-grain types.

Yearly environment affected the response of yield to planting date (Figure 2). In 2001, grain yield increased continuously as planting date advanced from the end of February through the middle of March, peaked with planting around the first of April, and declined continuously at each subsequent planting date. In 2002-2004, the patterns were relatively similar. In each year, grain yield was relatively constant with planting from the end of February through the first of April, peaking with planting around the middle of April, and declining with later plantings.

Overall, high milling (head rice) yield of main crop rice was associated with early planting, and milling yield gradually declined at later planting dates (Figure 3). Milling yield was uniquely low for two planting dates, the first of May and June. The trend was consistent for conventional long-grain and Clearfield types. Milling yields of medium-grain types were only dramatically reduced at the early June planting date. In contrast, hybrids exhibited no consistent effect of planting date on milling yield.

Ratoon crop production was evaluated following harvest of main crop rice planted from the end of February to the middle of April (Figure 4). Averaged across all genotypes, the main crop planted in the middle of March produced the highest ratoon yields, and ratoon yields declined continuously when the main crop was planted around the first and middle of April. The pattern of total crop yield (main plus ratoon yields) followed main crop yields for the planting dates from the end of February to the middle of April. Yields were relatively constant for the first three planting dates and increased when the main crop was planted in the middle of April. Main crop yields (6,000 to 7,000 pounds per acre) had a greater effect on total crop yields because they were higher than ratoon crop yields (1700 to 2200 pounds per acre).

Other agronomic characteristics were also affected by planting date. Seedling stands of the February planting were significantly poorer than of later planting dates, except for the July planting. The early season effects of planting date may be attributed to the low air and soil temperatures that occur during February that can cause slow germination and growth, and lead to increased seedling disease. The poor stand associated with the July planting date may have resulted from high water temperature following flood establishment. Because of steadily increasing soil and air temperature from February through July, the time between planting and seedling emergence decreases as planting date advances. The average number of days from planting to seedling emergence for rice planted in February was 19 days compared with 5 days for rice planted in July. For all genotypes, maturity (days between seedling emergence and 50% heading) was shortened as planting advanced from the end of February to the first of July. The average maturity was 90 days for rice planted in February and 70 days for rice planted around the first of June. For rice planted in July, a slight increase in maturity (74 days) was noted.

In summary, the optimum planting date for conventional rice types (long-, medium-, and short-grain and Clearfield varieties and experimental lines) was the middle of April. Hybrids performed best when planted around the first of May and consistently had higher yields than conventional rice types. Environment is a key factor and optimum planting dates can change yearly. Rice planted before the optimum date usually had a poor seedling stand but better milling quality and higher ratoon yields.

Rice planted after May 15 will suffer substantial losses of both grain and milling yields. When seeded between April 15 and May 15, long-grain and Clearfield rice types may have acceptable grain yield but may expect a significant head rice yield reduction, while such a reduction is much less for medium-grain rice types. Milling yield of hybrids was erratic and did not appear to be affected in a consistent manner by planting date.

Milling yield of hybrids was lower than milling yield in conventional rice types. Yield potential, as well as milling yield, as affected by planting date, is valuable to our rice growers because they often make decisions on which crop or variety to plant while considering commodity prices, production cost, and environmental conditions. These decisions are frequently considered in case of replanting or late planting when adverse weather conditions or crawfish production prevent rice establishment during the optimum seeding period.  

This research was partially supported by the Louisiana Rice Research Board.

Xueyan Sha, Assistant Professor, and Steve D. Linscombe, Professor, Rice Research Station, Crowley, La.

(This article appeared in the summer 2005 issue of Louisiana Agriculture.)