Scientists researching soybean planting dates and fertility rates for southwest Louisiana are pinpointing the optimal time to plant.

The planting dates in the study are divided into six time frames beginning in late March and extending into the late spring and early summer, said LSU AgCenter agronomist and rice specialist Dustin Harrell.

While the goal is to allow two-week intervals between the plantings, the dates are actually dependent on when the soil moisture is adequate and not excessive, he said.

“What we found in the date-of-planting study is the earliest planted beans were also the first to be attacked by insects and disease,” Harrell said. “In past years, we’ve found that planting mid-April to mid-May produced the best yields in the southwest region.”

In addition to the date-of-planting study, Harrell is also evaluating plant response to fertilization on soils limited in the nutrients phosphorus, potassium, sulfur and zinc. These trials consist of fertilizer rate and time of application treatments, he said.

“What we’re looking to find is the optimal fertilizer levels based on soil tests,” Harrell said. “Unlike the planting trials, which are consistently conducted on the south farm of the Rice Research Station in Crowley, the fertility studies are being conducted on a private farm in the Iowa area to evaluate nutrient response on different soils.”

In the southwest part of the state, where rice is the major crop, rice producers typically drill soybeans into flat ground instead of in beds because they have drills, not row planters, Harrell said.

“Most growers in southwest Louisiana are not equipped to pull rows, so they use the equipment they have,” he said.

These growers are at a disadvantage because soybeans grown on raised beds tend to do better than drilled beans, Harrell said.

In the southeast part of the state, AgCenter weed scientist Al Orgeron is looking at the benefits of rotating soybeans and sugarcane.

Orgeron said the main goal of the project is to help cane growers make a little extra money on fallow cane ground. With a typical cane crop, cane is on the field for five to six years, and then the ground is left fallow for about nine months, he said.

“During the months between the last cane harvest and the planting of a new crop, we try to get a crop of soybeans harvested,” Orgeron said.

Another reason for the rotation is weed control, which will be a problem whether anything is planted on the land or not.

During the 1970s, some sugarcane growers were rotating soybeans with their sugarcane, but they had a major problem with grasses that couldn’t be sprayed with glyphosate without killing the soybeans, Orgeron said.

Before the glyphosate-resistance gene, known as Roundup Ready, was introduced in soybeans in 1996, spraying Roundup to kill weeds would also kill the soybeans. Growers were in a no-win situation.

“The biggest weed problems we have in sugarcane are johnsongrass and bermudagrass,” Orgeron said. “But this Roundup Ready technology lets us spray Roundup, which takes out these two weed problems.”

Orgeron said the wet spring caused most of the cane growers to miss their opportunity to plant soybeans this year.

One of the struggles for growers is just the ability to get seed, he said. The seed companies don’t release seed until March, which makes timely planting extremely difficult, Orgeron said. In St. James Parish, only three growers were able to plant beans in rotation this year — and they were only able to plant about 25% of their acreage, he said.

“The timing of harvest is very important because research shows that every month beyond August that cane is not planted reduces the yield five tons each month of delay,” he said.

Another issue that sugarcane growers face is the need to inoculate the soybean seed so nitrogen fixation can occur. Soybeans obtain most of their nitrogen from the air when certain bacteria are present in the soil, and in the sugarcane-growing areas of the state there is not enough of the bacteria already in the soil, he said.

Johnny Morgan

LSU AgCenter agent Al Orgeron checks on soybeans that are rotated with sugarcane at the AgCenter Sugar Research Station at St. Gabriel. Orgeron said he is looking for varieties that are suitable for rotation with sugarcane. The soybeans have to reach maturity in time for the grower to plant cane in mid-August. Photo by Johnny Morgan

Soybeans being studied for optimum planting dates in southwest Louisiana. Photo by Bruce Schultz