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

A rice line that exhibits tolerance to the imidazolinone class of herbicides was released by the LSU AgCenter in 2002 and sold under the trade name Clearfield. Clearfield rice was developed through seed mutagenesis allowing rice lines to be considered nontransgenic.
 
Imazethapyr is labeled for use in Clearfield rice as Newpath at 4 to 6 ounces per acre applied to the surface as a preplantincorporated, preemergence or early postemergence treatment, followed by 4 to 6 ounces per acre postemergence applied 10 to 14 days later.

Three years after the introduction of Clearfield rice, another imidazolinone herbicide, imazamox, was labeled for use and sold under the trade name Beyond. This herbicide can be used late season for control of late-emerging weeds or as a salvage treatment without the potential of herbicide carryover to the following year’s crop.

In 2011, Beyond was labeled for use as an early-season herbicide choice to replace the second application of Newpath. By planting Clearfield rice and using Newpath and Beyond for weed management, this technology offered the opportunity to control red rice with no effect on the rice crop. Approximately 70 percent of the total rice acreage in Louisiana was Clearfield rice in 2010.

Red rice and other weeds compete with rice, and these weeds reduce grain yield and cause a reduction in milling quality and grade. Red rice is one of the most troublesome weeds of cultivated rice in the southern United States, and it has been recognized as a weed in U.S. rice fields for more than 150 years. Controlling red rice, which is genetically similar to rice, was not possible with traditional herbicides before the development of Clearfield rice.

In addition to red rice and barnyardgrass, a number of other grasses and broadleaf weeds cause problems in growing rice in Louisiana. The most common weeds include broadleaf signalgrass, ducksalad, hemp sesbania, spreading dayflower, alligatorweed, Texasweed and Indian jointvetch. Newpath and Beyond can effectively control many key grass weeds in rice, including red rice, barnyardgrass and broadleaf signalgrass. However, the use of Newpath provides minimal control of hemp sesbania and Indian jointvetch. Herbicide mixtures have proved to be beneficial in improving efficacy and broadening the weed control spectrum in Clearfield rice. The use of herbicide mixtures is favorable to producers because of the increased weed control and reduced application cost. Several herbicides can be applied at planting or through the three- to four-leaf stage of rice to allow establishment of the crop with minimum weed competition and provide residual control of weeds yet to emerge.

Several herbicides labeled for use in this manner include Command, Facet, Grasp and Prowl H2O. With this in mind, the objective of this project was to evaluate the economical effects of Command, Facet, Grasp and Prowl H2O applied with the first application of Newpath early postemergence to 1-leaf rice, followed by a second application of Newpath or Beyond 10 days later on Clearfield rice. Data from this study could prove to be essential when considering a herbicide with residual activity in a Clearfield production system.

A study was conducted at the LSU AgCenter Rice Research Station near Crowley. The long grain rice Clearfield 131 was planted at a rate of 75 pounds per acre. The herbicide programs included a herbicide with residual activity – Command at 12.8 ounces per acre, Facet at 0.67 pounds per acre, Grasp at 2.8 ounces per acres or Prowl H2O at 2.1 pints per acre – in mixture with Newpath at 4 ounces per acre applied early postemergence, followed by a single application of Newpath at 4 ounces per acre or Beyond at 5 ounces per acre applied two weeks later. A crop oil concentrate, Agri-Dex, was added in each application.

Economic analysis was 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 and 2 were zero. The price reductions for the other grades per hundredweight were: $0.25 (grade 3); $0.55 (grade 4); $1.25 (grade 5); $1.50 (grade 6) and $2 (sample grade). Newpath was priced at $525 per gallon, Beyond at $605.60 per gallon, Prowl H2O at $38.60 per gallon, Command at $138.15 per gallon, Facet at $56.80 per pound, Grasp at $1,362.60 per gallon and Agri-Dex at $15 per gallon. Profitability of the herbicide programs was determined by evaluating the total value product, which was calculated by multiplying the rice yield by the rice price of $0.13 per pound. Net returns above herbicide cost were also evaluated, where the net returns equal the total value of rice minus the herbicide program cost.

Compared with the standard program of two applications of Newpath, none of the soil-residual herbicides evaluated in this study increased Texasweed control. However, increased control of red rice, barnyardgrass and alligatorweed was observed with programs that included Facet or Grasp. This increase in control indicates the importance of incorporating herbicide mixtures to the standard Newpath program in Clearfield rice. The increase in broad-spectrum weed control with the addition of a herbicide with residual activity can be beneficial to producers by increasing weed control with little increase in herbicide cost and no increase in application cost.

Rice treated with the standard program had a rough rice yield of 5,520 pounds per acre, a milling yield of 65 percent whole over 71 percent whole plus broken rice kernels, and a rice grade of 3. Herbicide programs that included Command, Facet or Grasp followed by Beyond resulted in an increase in rough rice yield of 910 to 1,490 pounds per acre, compared with the standard program. However, no difference in milling yield or rice grade was observed for all herbicide programs evaluated. Little to no difference in yield was observed with Beyond applied following any herbicide with residual activity, compared with Newpath applied following an application of the same residual herbicide, except for Command. Herbicide programs that included Facet or Grasp increased rough rice yield, compared with Command or Prowl H2O programs. This research indicates that the addition of Facet or Grasp in mixture with Newpath followed by Newpath or Beyond resulted in increased rough rice yield because of the increased broadspectrum weed control observed with these herbicide programs (Figure 1).

Profitability of these herbicide programs can be determined by evaluating the total value product, which was calculated by multiplying the rough rice yield by the price of rice. Therefore, the impact of the herbicide programs evaluated on rough rice yield and quality will directly affect total value product. Also, the net returns above herbicide cost can be calculated by subtracting the cost of the herbicide program from total value product. The standard program resulted in a total value product of $717.60 per acre. The cost for the standard program was $35.05 per acre, resulting in net returns above herbicide cost of $668.75 per acre. Herbicide programs with Facet or Grasp resulted in an increase in total value product of $179.40 to $193.40 per acre, compared with the standard program. A similar trend was observed when evaluating the net returns above herbicide cost. Herbicide programs of Facet or Grasp increased the net returns by 21 percent to 23 percent (Figure 2), compared with the standard program. Also, with a given soil residual herbicide, total value product and the net returns above herbicide cost were similar when Beyond was applied as the second herbicide application compared with Newpath applied as the second herbicide application.

When comparing herbicide programs that included a herbicide with residual activity, total value product was greater with programs that included Facet or Grasp, compared with Command or Prowl H2O programs. These data indicate that Facet or Grasp in mixture with Newpath followed by Newpath or Beyond resulted in increased profits, even though cost of treatment increased. This increase in profit was due to increased weed control and higher rice yield (Figure 1) increasing total value product, which overcame the additional herbicide cost (Figure 2).

In conclusion, the addition of Facet or Grasp in mixture with Newpath applied early postemergence followed by Newpath or Beyond applied postemergence 14 days later proved to be beneficial in a total weed management program. However, with any given herbicide with residual activity, applying Beyond in the second herbicide application instead of Newpath resulted in little to no economic advantage, except with Command. Herbicide programs evaluated in this study resulted in higher rough rice yields and economic benefits when the initial application included Facet or Grasp, which maximized overall economic returns.

Increased weed pressure, even for a short period of time, decreases rice yield. Therefore, producers should treat weed problems early, thus reducing weed competition, which allows rice to produce higher yields and, thus, higher profits. In this study, economic returns were increased by 21 percent to 23 percent when Facet or Grasp was added to the first application of a standard Newpath program.
 
Eric P. Webster, Florence Avalon Daggett Professor in Rice Research, and Tyler P. Carlson, Graduate Research Assistant, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; and Michael E. Salassi, J. Nelson Fairbanks Professor, Department of Agricultural Economics & Agribusiness, LSU AgCenter, Baton Rouge, La.

(This article was published in the summer 2011 issue of Louisiana Agriculture magazine.)