Louisiana Rice: Cold-Stunted Plants Complicate Flooding Decisions

Most of the rice in southwest Louisiana went into the ground during a two-week interval when we had warmer weather and dry soil. When most of that rice emerged in mid-March, the weather conditions became cool and slowed crop development down considerably.

Normally, after about a 4-week period after emergence, the rice will generally reach that 5th-leaf to first-tiller stage of development and we are ready to establish the permanent flood. However, for most of our mid-March emerged rice, that is not the case this year. Most of the mid-March emerged rice is stalled at that 3-leaf stage of development. Similarly, the rice that emerged the first of March is stalled at or just before tillering.

With the extended cold temperatures, we are just not getting the heat units we need to for the rice to develop at a rate we are used to this time of year.

Rice growth and development can be estimated by looking at the accumulated heat units after emergence. The number of heat units accumulated each day above 50 degrees F (with adequate soil moisture and no other limiting factors) can be used to estimate rice growth and development. These heat units are often referred to DD50 heat units, which is the backbone of the DD50 program. DD50 heat units can be calculated by getting the average daily temperature and subtracting 50.

In general, it takes between 75 to 175 heat units for each subsequent leaf to emerge during the seedling stages of rice development. This is variety dependent of course however, we can estimate that on average it takes about 150 heat units between the emergence of each leaf. Using this criteria, most of our mid-March emerged rice this year is around that 3rd leaf stage of development when normally we would be near tillering. I would estimate that we are about 300 heat units behind normal crop development this year.

Another thing we are seeing this year from our late February and early March planted rice are plants that are beginning to tiller that are very short in stature. In figure 2, you can see that this rice plant is already tillering and the overall stature of the plant is only around 4 inches tall. This phenomenon is present everywhere in south Louisiana this year and it most definitely related to the extended cool weather conditions. It has been seen in multiple varieties and hybrids as well. This in itself is not a yield limiting concern in rice.

In fact, one grower told me that he seems to remember in years with cold early season conditions generally correspond to the higher yielding years. I have heard the same thing from growers speaking about dry years. Let’s hope that this holds true for 2018 as well!

One concern of the short stature plants is the establishment of the flood. With short plants and high winds, water can stack up and submerge these short plants. Generally, rice will quickly stretch and grow out of submerged situations however, and with the cool temperatures this may be a much slower process this year. With warmer conditions moving back into the forecast for late next week we should be fine moving forward.


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