Earlier research, says Eure, revealed that in a cantaloupe-cotton intercropping system, an ethalfluralin plus fomesafen approach applied pretransplant gave adequate Palmer amaranth control.

“We have the potential for cantaloupe injury, but when we intercrop we don’t see a reduction in cantaloupe yield. We did see a reduction in cotton yield of about 14 percent, but we were still able to increase our growers’ profitability per acre by 18 percent compared to cantaloupes grown in a monoculture. So we wanted to do something very similar with watermelon-cotton intercropping.”

Trials in two separate years included three herbicide systems: ethalfluralin pretransplant, ethalflualin plus fomesafen pretransplant, and ethalfluralin plus fomesafen plus terbacil pretransplant.

“We also had a weed-free monoculture of watermelon and cotton. We had two planting dates for cotton because our intercropped cotton was planted three to four weeks earlier than we prefer to plant in Georgia. We also had an ideal planting date of May 10-15 for cotton.”

Pretransplant herbicides were broadcast over the entire plot area, resulting in herbicide residue on the plastic mulch, he says. This residue has been known to cause transplant injury to seedling watermelons, so overhead irrigation was used to rinse the plastic mulch which also activated pretransplant herbicides.

“We transplanted our watermelon on 72-inch centers. When our watermelon vines reached the edge of the plastic mulch, we came in and planted our cotton on 36-inch spacing. This was about 18 to 21 days after transplanting our watermelons.

“It’s important to understand that we managed our intercropping system for watermelon production until harvest. That’s because we had about $2,900 per acre invested in our watermelon crop, significantly more than the investment in our cotton crop.”

Watermelons were harvested two to three times in the studies, beginning the third week of June and finishing by July 4.

“Once we were done with watermelons, we burned down the vines with glyphosate followed by glufosinate seven days later. Then we lay by with diuron plus MSMA prior to canopy closure. We left the plastic mulch down until we were done harvesting cotton. We recorded pigweed control, crop injury, watermelon vine length, cotton height and yield, and we calculated our crop value per acre.”

Watermelon growth and development wasn’t influenced by intercropping or by herbicide systems, and that was reflected in watermelon vine length, says Eure.

“Looking at pigweed control prior to watermelon harvest, when we used ethalfluralin alone, we had poor pigweed control. When we included fomesafen in our sysem, we increased our control to 91 to 95 percent. Our non-treated control had an average of 160 Palmer amaranth plants. When we included fomesafen in the system, we decreased that to five or six per plot. It’s not perfect, but it’s better than what we’re used to.”

When the fomesafen-based herbicide system was used, watermelon yields were similar to the monoculture system, he says. When ethalfulralin was used alone, there was poor weed control and a reduction in watermelon yield.

“In systems where we used terbacil, we ended up causing complete crop death in cotton. The crop came up, but about two weeks after emerging we started seeing symptoms, and about six weeks later the plants were dead. The label for terbacil has an 18-month rotation restriction, and this clarified that.”

Looking at cotton height prior to harvest, there was a difference between the two planting dates mainly because intercropping system dates were two to three weeks earlier than ideal planting dates, he says. When the ethalfulralin plus fomesafen-based system was used, there was a reduction in cotton height compared to monoculture, showing some level of competition between the two crops.

“When we use ethalfulralin alone, pre-transplant, and follow it with glufosinate and glyphosate to kill our vines, we have poor weed control late in the season. But when we use fomesafen in the pretransplant system and maintain early season Palmer amaranth control, we’re able to maintain good palmer amaranth control later in the season.”

Even though there was a reduction in cotton yield when intercropped with watermelon, there was a positive effect with crop value per acre, says Eure.

When calculating crop value, researchers use a value of 17 cents per pound for watermelon with variable production costs of $2,900 per acre. For cotton, the value was 82 cents per pound with variable costs of a monoculture system of $565 per acre. Those variable costs were reduced to $365 per acre in an intercropped system due to an overlap in resources.

“When we used the ethalfulralin plus fomesafen-based system for good Palmer amaranth control, we increased our crop value per acre by 14 percent compared to watermelon grown in a monoculture, and we increased it by more than 1,000 percent when compared to a cotton monoculture system.

“We have excellent crop tolerance with an ethalulralin plus fomesan-based system, and we had good but not perfect Palmer amaranth control. We had some competition between the crops, but watermelon was not influenced. Cotton yield was reduced, but we were still able to increase our crop value per acre by 14 percent when compared to monoculture watermelon production and greater than 1,000 percent when compared to a cotton monoculture production.”

Intercropping, says Eure, can help growers be profitable when they can control glyphosate-resistant Palmer amaranth. He notes that fomesan isn’t currently registered for use in watermelon, nor is ethalfulralin registered for use in cotton.

“For future research, we’ll continue to look for other herbicide options, develop integrated management systems for our growers, and we’ll continue to evaluate other intercropping systems.”