Good weed control starts with proper weed ID

Vicky asked me to talk about weed management, so I asked Dr. Muthukumar Bagavathiannan to help me. Muthu is our rice weed scientist located at College Station, but he makes many forays into the Texas Rice Belt during the growing season.

Of utmost importance is proper weed identification, which demands frequent and careful scouting.

Last year, I was called out to identify a prostrate, broadleaf weed infesting a Chambers County rice field before flood. I brought it to the greenhouse and reared it to flowering, at which time I was able to identify it as Virginia buttonweed. Farmers east of Houston should be on the lookout for this invasive weed in 2019.

If you don’t practice no-till, plant into a well-prepared seedbed devoid of seedling weeds. For no-till, make sure all weeds are dead or dying before you plant. Apply burndown herbicides/combinations that are effective against the entire spectrum of weeds (including resistant ones like pigweed) in your fields. And remember, effective control of early season weeds is a key resistance management strategy.

Apply a pre-emergence, residual herbicide (e.g. clomazone), but be careful to adjust the rate based on your soil type. As you know, heavy soils bind herbicide molecules, so these soils require higher rates of soil-applied herbicides compared to lighter soils.

Refer to product labels for recommended rates specific to your soil type. Labels can be downloaded from www.cdms.net or www.greenbook.net. If there is no projected rainfall within a couple of days of application, flush immediately to activate the herbicide. About 0.5-1 inch of water is sufficient for activation.

In my own research plots, I have observed severe stunting, bleaching and phytotoxicity when I applied too much clomazone. Also, I think this stunting and phyto persist longer when conditions are cold and wet, which often are associated with early plantings.

If you apply a post-emergence herbicide, make sure you apply it when weeds are small. Once grass weeds begin to tiller, they are very difficult to control, and they aggressively compete with rice for space, sunlight, water and nutrients.

To us, applying post-emergence herbicides too late seems to be the most common mistake in weed control. We know that trying to schedule an aerial application in the spring is difficult, to say the least. Rain, wind (speed and direction) and irrigation scheduling can play havoc with your best-laid plans. In spite of all the technological advances in rice production, we still are at the mercy of Mother Nature.

We may need to rethink our N program for row rice

Managing nitrogen fertilizer in a row-rice environment will be a little different than what we as Mid-South rice producers have been accustomed to in our direct-seeded delayed-flood production environment.

Traditionally in flooded rice culture, we apply two-thirds of our nitrogen at the five-leaf stage, followed by establishment of the flood. Our remaining N is applied either at midseason or late boot, depending on our cultivar selection. This program is the most efficient strategy in flooded culture but may not be as efficient in furrow-irrigated rice, depending on the environment. Therefore, a change will more than likely be required when managing N in a furrow-watered situation.

When considering N management in furrow-irrigated rice, a few things have to be considered. The first is am I going to stop up my discharge pipes and allow a portion of the field to remain flooded? Second, how great is my slope?

On row-rice fields where the bottom of the field will be stopped up, there will be three distinct management zones with varying levels of flood depth after the first irrigation. On these fields, split applications of nitrogen are recommended.

If the slopes are not that great (< 0.1 inch fall), we may be able to get by with a more traditional program, especially on soils that have a high clay content. However, because there are distinct zones in the field, we may have to make additional applications in the zone with less standing water.

The safest (from an N loss standpoint), but more costly, recommendation is to use a “spoon feeding” approach to nitrogen management on row-rice fields. Preliminary research in the upper Mid-South rice-producing states suggests a spoon-feeding strategy can produce great yields on row-rice fields with steep slopes and/or fields where producers have difficulty maintaining a uniform flood.

The spoon-feeding strategy that has provided great yield potential in Mississippi has been to apply N in four split applications, with each application representing about 25 percent of the total required nitrogen rate. The first application should be made at the five-leaf stage, with subsequent applications every seven to 10 day in front of an irrigation.

In practice, the spoon-feeding method on Mississippi farms has equated to applying 100 pounds of urea in three to four applications, with the fourth application many times only conducted on the top side of the field.

If you have questions on fertilization of row rice, give us a call and we will help discuss an appropriate N strategy for your row-rice situation.

Zinc and the kitchen sink

High pH soils in Arkansas can routinely have zinc (Zn) fertility issues. The most common problem fields are generally silt loam or sandy loam soils and have a pH greater than 6.0. Soils fitting this description need added Zn if the soil test indicates less than 4 ppm of Zn.

Recent research by Dr. Nathan Slaton continues to show the advantages of 10 pounds of actual Zn applied as zinc sulfate.
Compared to many other products at various rates, zinc sulfate at 10 pounds of actual Zn does the most to increase Zn tissue concentration in seedling rice.

Other products may contain Zn but typically at rates much lower if applied based on product recommendations. The added benefit of 10 pounds Zn is that we can build the soil test level and reduce our application needs in the future.

Zinc seed treatments also have value to rice growers. However, they are not a replacement for proper Zn fertility as described above. The current recommendation is to have 0.25 to 0.5 pound Zn per hundredweight (cwt) as a seed treatment. But recent surveys have suggested we are not always getting that amount on the seed.

A Zn seed treatment will help to iron out some of those severe problem areas with extremely high pH and low Zn soil levels. In situations with high pH and low soil test levels, Zn seed treatments do not provide sufficient Zn alone.

There has been a great deal of talk lately about new urease inhibitors for rice. All these new products contain NBPT, generally providing very similar reductions in ammonia volatilization and increases in nitrogen (N) efficiency. At this time, I have no reason to recommend one of these products over another.

One item to note is that if you are able to flood a silt loam in three days or less or a clay in seven days or less, you don’t need NBPT-treated urea. Just plain urea will do. Having said that, if you have any flood establishment concerns, NBPT-treated urea is cheap insurance to protect your N investment.

GreenSeeker helps assess mid-season top-dress nitrogen need

Early planting generally leads to higher yields with an average 1.5 100-pound sacks-per-acre yield penalty for every week delay during May. That said, you do not want to cut corners on sound management practices to simply plant a bit earlier.

One thing I learned in 2018 doing the yield contest is the importance of getting a uniform stand of seedlings. You want to see a uniform 20 to 25 seedlings per square foot for optimal performance.
Seeds can tend to bunch together due to poor seed bed preparation or winds during and shortly after planting. By panicle initiation (PI), you would like to see 70 to 80 tillers per square foot.

Despite an increasing trend to apply a top-dress nitrogen application at panicle initiation, these applications may not be necessary. Top-dressing N is expensive as it requires an airplane and a more expensive form of N fertilizer.

Top-dress N applications are needed if the plant is N deficient or you suspect it will become so. At PI (45 to 55 days after planting), the crop should be assessed to determine if a top-dress of nitrogen fertilizer is necessary.

A good assessment is important because not applying N when needed can lead to a reduction in yield; however, applying N fertilizer when it is not needed can lead to lodging, delayed maturity, increased incidence of disease and reduced yields.

The GreenSeeker, which measures the NDVI (Normalized Difference Vegetation Index) of the canopy, is a new tool that we have been testing.

We have found that a response index of 1.1 or greater indicates the need for top-dress N application.

The response index is the NDVI reading of an enriched N strip (representing a crop with unlimited N) divided by the NDVI reading from the field test area.

The enriched strip is an area where extra N was added to the field (could be done by overlapping an area with an aqua rig or a small area where you intentionally added extra N).

For example, if the N enriched strip gave an NDVI value of 75 and the field test area gave an NDVI value of 68, the response index would be 1.1 (75/68=1.10) and this would indicate the need for a top-dress N application.

I mentioned above that over application of N can lead to increased disease. In 2018, we saw an increase in kernel smut. High levels of kernel smut are associated with high nitrogen levels.

Kernel smut can lower both yields and grain quality. The variety, M-209, seems more susceptible to kernel smut than some of our other medium grains, so be particularly mindful of over applying nitrogen fertilizer if you are growing that variety.

Additional information on nutrient management as well as other aspects of crop management can be found at the UC Rice website, http://rice.ucanr.edu/.