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Linda Benedict, Webster, Eric P., Salassi, Michael  |  2/28/2011 10:16:55 PM

Figure 1.

Figure 2.

Propanil mixed with Newpath increases Clearfield rice yields, if weeds are treated early

Eric P. Webster, Tyler P. Carlson and Michael E. Salassi

Weed management decisions in rice often drive the overall production system. Economic considerations determine the specific herbicides a producer will include in a weed management program. The adoption of a weed control program depends on saving money, maximizing yield or reducing inputs. Other factors important in weed management decisions are perceived simplicity, manufacturer incentive programs and crop safety.

Red rice is one of the most troublesome weeds in Louisiana rice production and has been recognized as a weed in U.S. rice fields for more than 150 years. However, in 2002, imidazolinone-resistant rice, which was developed at the LSU AgCenter Rice Research Station and sold under the trade name Clearfield, became commercially available. This offered the opportunity to effectively control red rice with no effect on the crop. In 2010, approximately 70 percent of the total rice acreage in Louisiana was Clearfield varieties.

The main herbicide used in Clearfield rice is imazethapyr, sold under the trade name Newpath. It provides broad-spectrum weed control with both soil and foliar activity. Newpath has excellent activity on red rice and other grass weeds, but its activity is inconsistent on other weeds such as yellow nutsedge, hemp sesbania and Indian jointvetch.

Mixing two or more herbicides – often referred to as mixtures or tank mixes – has been beneficial in improving efficacy and broadening the spectrum of weed control. For many years, the weed control program for rice in the South centered on the herbicide propanil, which has long been used to control annual grass and broadleaf weeds. Several formulations of propanil include Stam M4, Stam SC, Stam EDF, RiceShot and SuperWham. With this in mind, the objective of this study was to evaluate the economic effects of these various propanil formulations with Newpath applied at early postemergence or late postemergence in Clearfield rice production. 

The study was conducted at the study at the Rice Station, near Crowley, to evaluate various herbicide programs. The long-grain rice variety Clearfield 131 was planted at a rate of 75 pounds per acre, and the herbicide programs included: A propanil formulation in mixture with Newpath applied early postemergence (after the plants emerged from the soil) followed by Newpath alone applied late postemergence. Newpath alone applied early postemergence followed by Newpath plus a propanil formulation applied late postemergence. The propanil formulations evaluated were Stam M4, Stam SC, Stam EDF, RiceShot or SuperWham at the equivalent of 3 quarts per acre and Newpath at 4 ounces per acre. A crop oil concentrate, Agri-Dex, at one percent by volume was added in each application, except for Stam M4 and RiceShot, which are formulated so crop oil is not recommended. Economic evaluations were based on an average long-grain rice price of $0.13 per pound with price deductions based on rice grade. Actual rough rice market prices are adjusted by grade, and these grade price discounts can vary across rice mills. In this study, rice price deductions for grades 1, 2, 3, 4, 5, 6 and sample grade were zero, zero, $0.25, $0.55, $1.25, $1.50 and $2 per hundredweight, respectively. Newpath was priced at $525 per gallon, Agri- Dex at $15 per gallon, Stam M4 at $25 per gallon, Stam SC at $37 per gallon, Stam EDF at $15 per pound, RiceShot at $30 per gallon and SuperWham at $33 per gallon. The profitability of each herbicide program was determined by evaluating the total value of the product, which was calculated by multiplying the rice yield by the rice price. Net returns above herbicide cost also were evaluated, with the net return equal to the total value of rice minus the herbicide cost.

The standard program included Newpath at 4 ounces per acre applied early postemergence followed by Newpath at 4 ounces per acre applied late postemergence. When any propanil formulation was added at 3 quarts per acre in either first application of Newpath, red rice control increased compared with the standard program. Throughout the growing season, this trend was observed on red rice, barnyardgrass, alligatorweed and texasweed. From a weed control standpoint, SuperWham, RiceShot and Stam EDF appeared to be the better propanil formulations if used with the second application of Newpath.

Regardless of timing, weed control with a propanil-Newpath mixture was equivalent to or higher than the standard program of two Newpath-only applications. The addition of propanil, regardless of formulation, also was observed to be more effective for weed control when added in the first application of Newpath instead of the second application. This increase in control indicates the benefit of incorporating other herbicides in a mixture with Newpath to maximize weed control across multiple weed species.

Rice yield, milling and rice grade were determined. Rice treated with the standard Newpath program had a yield of 3,810 pounds per acre (Figure 1). The percent whole rice kernels over percent whole plus broken rice kernels indicated that the standard Newpath program resulted in a milling yield of 65/71 with a rice grade of 3. Rice treated with propanil, regardless of formulation, in the first Newpath application or with Super- Wham in the second Newpath application resulted in yield increases of 1,270 to 2,650 pounds per acre, compared with the standard program. However, no differences in milling yield and rice grade were observed.

In every case, rice treated with any propanil formulation mixed in the first or second application of Newpath increased rice yields compared with the standard treatment. Rice yields increased more when propanil was added to the first Newpath application compared with adding propanil to the second Newpath application. Previous research has shown that controlling weeds in rice within the first three to four weeks after emergence increases rice yields. This is why adding propanil in the first application of Newpath is more beneficial. The weeds are smaller, and control is achieved before competition from weeds can reduce rice yields.

Profitability of these herbicide programs can be determined by evaluating the total value, which was calculated by multiplying the rice yield by the riceprice. Therefore, the effect of propanil in a mixture with Newpath on rice yield and quality will directly affect total product value. Also, the net returns above herbicide cost can be calculated by subtracting the cost of the herbicide from total product value (Figure 2).

The standard Newpath program returned a total product value of $490 per acre and a net return of $450 per acre.

Programs that included propanil with the initial Newpath application or SuperWham with the second application increased total product value by $160 to $340 per acre compared with the standard program. Programs that included propanil in the initial Newpath application increased the net returns above herbicide cost by 29 percent to 71 percent compared with the standard program. The addition of propanil in the second application of Newpath increased net returns above herbicide cost by 11 percent to 42 percent. Even though total product value was increased with Stam SC, the overall increase was lower for each timing because of the increased herbicide cost. A mixture of Newpath plus RiceShot in the first application gave a 71 percent increase in returns over herbicide cost compared with the standard treatment. These data indicate that adding propanil in with Newpath increases profits, even though treatment costs increase. This increase in profit was due to increased weed control and higher rice yield, which increased total product value and made up for the additional herbicide cost.

Adding propanil in mixture with Newpath proved to be beneficial in a total weed-management program. The herbicide programs evaluated in this study resulted in higher rice yields and net returns when the early postemergence application included a propanil formulation. A herbicide program that included RiceShot or Stam M4 in the early postemergence application maximized overall economic returns. When propanil was applied in the late postemergence application, however, overall economic returns were maximized with SuperWham.

Increased weed pressure, even for a short time, decreases rice yield. Therefore, producers should treat weed problems early. When weeds are controlled early, weed competition is reduced and yields are increased, producing higher profits.

Eric P. Webster, Florence Avalon Daggett Professor in Rice Research, and Tyler P. Carlson, Graduate Research Assistant, School of Plant, Environmental & Soil Sciences; Michael E. Salassi, J. Nelson, Fairbanks Professor,Department of Agricultural Economics & Agribusiness.

(This article was published in the winter 2011 issue of Louisiana Agriculture Magazine.)

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