Arkansas soybean farmers who want to plant early maturing group IV varieties and even late group IIIs in April and early May may need to modify their seeding rates and row spacings, says Dr. Lanny Ashlock, soybean specialist for the Cooperative Extension Service, University of Arkansas.

Ashlock and other UofA scientists are conducting a study to determine which seeding rates and row spacings are best for early plantings of the early maturing varieties. The work is financed by grower checkoff funds granted by the Arkansas Soybean Promotion Board.

"We felt that we might need to increase the seeding rate, but we weren't really sure how important row spacing was," said the specialist.

Ashlock's dryland research plots are in sandy loam fields, one on Charlie Starks' farm in Miller County and the other at the UofA Pine Tree Branch Station in St. Francis County.

"We planted four varieties at four different seeding rates and three row widths," said Ashlock. "The varieties used were Manokin, Northrup King RA 452 and Asgrow 4715, which are maturity group IVS, and Williams 82, which is a late group III.

"We planted each at rates of 30 pounds of seed per acre, which translates into about 60,000 plants per acre; 60 pounds of seed, or about 120,000 plants per acre; 90 pounds of seed, or 180,000 plants per acre; and 120 pounds of seed, which produces about 240,000 plants per acre."

Ashlock noted that most farmers plant 50 to 60 pounds of seed per acre on 30-inch rows for their conventional May and June plantings of group IV, V and VI varieties. "When they go to narrow rows, many farmers automatically raise their seeding rate to 75 to 90 pounds per acre.

"What we wanted to find out with our research was whether we needed to modify our current seeding recommendation for earlier planted late IIIs and IVS."

In Ashlock's study during 1995, which was a dry year, seeding rates of 30 to 60 pounds per acre were adequate regardless of the row spacing. (The research plots had 10-inch, 20-inch and 30-inch rows.)

"The best yielding plant populations in the research plots were the lower to medium rates of 60,000 and 120,000 plants per acre," the specialist said. "Generally, the more we increased the seeding rates during the dry year, the lower the yields.

"Our earlier work showed that you need to have a minimum of 60,000 plants per acre with the determinate group V and VI varieties. We believe the optimum plant population for these determinate varieties is 100,000 plants per acre. That's our goal.

"If a farmer plants 50 pounds of a group V or VI, he's putting out about 150,000 seeds with hopes of having 100,000 plants. Normally, he'll have a few more plants than that."

Ashlock's research on sandy loam soil indicates that, in both dry and wet years, the seeding rate doesn't have much effect on yields as long as you have a minimum of 60,000 plants per acre in an even stand.

"Soybeans, even many of the indeterminate varieties, have a way of compensating. If the stand is thinner, the plants will become more bushy and fill in the space."

He said, "Row spacing seems to have more of an effect on the yields of very early planted soybeans, especially during wet years."

Ten-inch to 20-inch rows tend to be better than 30-inch rows for April plantings, according to Ashlock. "As long as the plant population isn't excessive, the narrower the row spacing, the more efficiently plants can use nutrients and the better they can compete with weeds.

"These early planted, early maturing varieties can really be hurt by late season weed pressure, especially with row spacings of 30 inches or wider.

"Planting on wide rows sometimes hurts yields, too. These very early maturing indeterminate varieties flower so quickly that if you have wide rows, the plants will never lap the middles.

"So if you're going to plant very early maturing varieties in April, row spacings of 20 inches or less will more consistently maximize your yield potential.

"Row spacing has not been as critical on our alluvial silt loam plots once we got into May and June plantings."

Ashlock also included a group V variety, Hutcheson, in his study. "I found that planting a determinate variety that early didn't

reduce it's yield potential, but it did reduce plant height, which can be a problem when you're harvesting.

"Plus, planting the group V variety early didn't move up maturity that much. It matured within a week of when it would have if it had been planted in May."

UofA agricultural engineer Dr. Earl Vories is doing research similar to Ashlock's on heavy clay soil at the university's Northeast Research and Extension Center, Keiser. "When we looked at 7.5-inch and 10-inch row spacing, we saw a yield response to seeding rates higher than our recommendation.

"When you plant on heavy clay, you don't get as big a plant as you would planting in a sandy loam soil. There's less vegetative growth, so there's room for more plants in the field."