Arkansas rice growers seek cause of low yields in 2005

A weed scientist's job usually is finished by this time of year. However, glyphosate drift and misapplications (such as tank contamination) have forced me out into the field this year later than ever.

In a growing trend, I have had numerous calls this year that started out, “We made a couple of rounds with the combine and realized something was wrong.”

Glyphosate damage can be difficult to detect until seedhead emergence in many fields. By that time, many folks are almost finished walking rice fields. So, it is understandable how it can be missed.

However, glyphosate or other herbicide drift missed until harvest can make it impossible to tell exactly what happened (or where it came from). This year, I have learned that rice is sensitive to glyphosate even just prior to seedhead emergence or even during seedhead emergence. We have observed several fields where exposure to glyphosate at heading has resulted in the seedhead not fully emerging from the sheath. This results in severe disease forming in the moist un-emerged seedhead.

However, when you open the sheath, the familiar “parrot-beaked” and curled panicles are visible on the lower part of the seedheads. As with other timings, this results in severe yield reductions.

According to what rice agronomist Chuck Wilson and I have heard, some varieties, such as Clearfield 161 and Wells, generally did not cut as high yields this year as last. Other varieties, such as the RiceTec hybrids, Francis and others, seem to have had a pretty good year.

In many cases low yields are being attributed to glyphosate, Newpath or some other herbicide drift or misapplication. That is to say growers are pointing to this first. However, this is likely not the case for all fields.

From a weed science standpoint it has been a tough year. I received more calls on late weed control in rice this year than in the past three that I have been with the Extension Service. In fields where we have to rely on pre-flood and post-flood applications of herbicides, competition from barnyardgrass and other weeds has likely caused irreversible yield losses. Even though the field was fairly weed-free at harvest, weed competition earlier in the season might have robbed yields.

Another factor may be disease if you have low yields and did not use a fungicide or are concerned about your fungicide timing. According to Rick Cartwright, our Extension pathologist for rice, many fields of Clearfield 161 and other varieties that are not yielding as expected may not have received a properly timed fungicide application. We know that with CL 161 and CL 131 a fungicide will almost always be necessary.

Finally, water management is hurting our overall state rice yields. Because of the drought this year, many fields were not flushed or flooded as needed. This can influence weed control, fertility, insect damage, disease and eventually yield.

Typically yield losses from glyphosate drift (or accidental applications to the field) can run 75 to 100 bushels. When yields are off by this much, I tend to suspect glyphosate drift. However, when yields are off by 40 bushels or less, one of the factors mentioned above is as or more likely.

With varieties like CL 161 you do not have a lot of room to spare in terms of yield loss, so even a minor reduction may be unacceptable. Before you make up your mind, consider all the management challenges in that field this year.

Bob Scott is the University of Arkansas Extension weed specialist. e-mail: [email protected]

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