Donnie K. Miller, Bill J. Williams and Steve T. Kelly

Staple is a selective herbicide labeled for both preemergence and postemergence control of broadleaf weeds in cotton. Since introduction in 1995, it has been used widely in the cotton-producing region of northeast Louisiana. During this period, lower cotton market prices have resulted in a larger portion of these acres being planted to corn, and producers have observed benefits to the cotton crop when it is rotated with corn.

Research has shown that rotation to corn following cotton for a two-year period can significantly reduce reniform nematode populations that reduce cotton yields. A potential problem on corn, as a result of this shift, is possible carryover of herbicides applied on the cotton crop. When we began our research, the label for Staple specified that corn could be planted nine months after application, with specific application and tillage restrictions. According to the label, this interval was provisional upon the application being on a directed band not to exceed 50 percent of the cotton row width and fields having a thorough soil mixing such as two diskings or a deep plowing before planting corn. Several questions were raised as to whether label provisions were too conservative.

To alert producers to the possible effect from applications not consistent with the herbicide label, research was initiated to evaluate corn tolerance to Staple using simulated carryover and rotational plantback studies over a three-year period beginning in 1998 at the Northeast Research Station in St. Joseph, La. These studies were conducted on a silt loam soil with 1 percent organic matter and a pH of 6.5. In all studies, corn plots were treated with Lorsban insecticide and a Prowl/atrazine herbicide tank mix. Recommended corn production practices were followed.

To assess possible negative effects of the Staple treatments, visual assessment of plant discoloration 21 days after planting, plant population and height 41 days after planting, plant height and 100-seedweight at harvest, and yield were determined.

Simulated Carryover
For the simulated carryover study, Staple was applied at 2, 1, 1/2, 1/4, 1/8 and 1/16 times (x) the normal use rate of 1.2 ounces per acre with a nontreated control included for comparison. Herbicide was incorporated to a depth of 2 inches immediately preceding planting of Pioneer 3223 corn on March 22, 1999, and April 10, 2000.

In 1999, discoloration of corn plants, as observed 21 days after planting for the respective rates of Staple evaluated, was as follows: 61 percent, 45 percent, 31 percent, 28 percent, 12 percent and 10 percent. In both years of the study, rates of 1/4x or higher were required to reduce plant population significantly compared to plots receiving no Staple. With respect to plant height 41 days after planting and at harvest, only the lowest rate of Staple was equivalent to the nontreated control in 1999. All other rates resulted in at least a 12 percent height reduction.

In 2000, rates of 1/4x or higher were required to reduce plant height significantly at harvest. The 100-seedweight was not affected by Staple rates evaluated either year. Yield reduction of at least 47 percent and 43 percent was observed with rates of 1/4x and higher in 1999 and 2000, respectively. Rates of 1/8x and 1/16x resulted in yields equal to the respective nontreated controls (182 and 145 bushels per acre).

Rotational Plantback
In the rotational plantback study, Staple was applied at one (1x) or four (4x) times the normal use rate of 1.2 ounces per acre broadcast underneath cotton plants over the entire width of each row on June 18, 1998, and July 6, 1999. A nontreated control was included for comparison. Plot rows then were only re-hipped (re-formed) immediately after cotton harvest and before corn planting in the spring on the above-mentioned dates, which correspond to nine months after Staple application.

In 1999, corn response in plots treated with Staple at 1x or 4x rates the previous year was not significantly different from those receiving no Staple. In 2000, significant differences were noted only with respect to plant height at harvest and yield. Corn height at harvest was reduced 19 percent when treated with the 4x rate of Staple compared with those in the 1x and nontreated plots, which was not different. A stepwise reduction in yield was observed for Staple at 1x and 4x rates (7 percent and 34 percent, respectively), from the nontreated control (152 bushels per acre), thereby supporting current label recommendations for band application followed by disking and thorough mixing of soil for a nine-month rotational interval to corn. Yield differences were not attributable to differences in plant population or 100-seedweight.

Variation between years may be because 45 inches of rain was received from the time of Staple application to corn planting in 1999, compared with only 32 inches for the same period with the 2000 corn planting. In addition, average daily temperature for the winter was slightly lower for 1999-2000 compared with 1998-1999. Staple degrades slowly in soil, primarily by microbial activity, with an estimated half-life (time required to reduce initial concentration by half) of 60 days in laboratory studies. Any environmental conditions that slow microbial activity, such as drier, colder conditions, may affect the degradation rate of Staple. This may explain the negative effects observed on corn for Staple carryover in the 2000 trial, thereby justifying the conservative label restrictions.

Follow Label Restrictions
Staple rates of 1/4x or higher were required to cause significant yield reduction when applied and incorporated immediately before corn planting. Given the estimated half-life in laboratory studies of 60 days for Staple, theoretically a six-month period would be needed to reach this level with an initial application rate of 1.2 ounces per acre. However, under variable environmental conditions, the time required may be much longer.

When applied in a manner inconsistent with the label, Staple herbicide at the normal use rate in this study affected corn yield negatively in one of two years. Effects were more pronounced under drier, colder conditions following application until corn planting, indicating a greater potential for carryover effects from off-label applications when such conditions exist.

Results from the plantback study indicate that the nine-month interval would not have been sufficient to provide complete crop safety with the removal of application and soil-mixing restrictions. Staple application and seedbed preparation were a worst-case scenario for Staple carryover potential. Staple was broadcast underneath cotton plants over the entire row width, instead of half the row width in a banded application. Therefore, when rows were only re-hipped and soil was not thoroughly mixed, treated soil from the entire row width could have been concentrated on the re-hipped row. Corn was tolerant to as much as a 4x rate of Staple in one of two years of the actual carryover study. Producers are cautioned always to adhere to label plantback restrictions because planting before the indicated interval, such as 30 days in our research, may lead to yield reduction in a subsequent corn crop.

Since initiation of these studies, the Staple label has been changed to specify that corn can be planted in Louisiana 10 months following application in cotton, if the amount applied does not exceed a total of 1.8 ounces broadcast per acre per season. No additional soil mixing (disking or plowing) is required beyond what is normally done with the various production systems (conventional tillage, minimum tillage, no-till or ridge till).

Acknowledgment
Donna R. Lee, A. Lawrence Perritt and Charles F. Wilson, research associates at the Northeast Research Station, for their work in the completion of this research.
 
Donnie K. Miller and Bill J. Williams, Assistant Professors, Northeast Research Station, St. Joseph, La.; and Steve T. Kelly, Assistant Specialist (Weed Science), Scott Research and Extension Center, Winnsboro, La.

(This article appeared in the winter 2002 issue of Louisiana Agriculture.)