Library Categories


Zero in on seeding rate

With high-priced seed, the economic consequences of planting soybeans at a rate higher than needed have never been greater. With high-priced soybeans, the consequences of losing yield because the final stand was too thin have never been greater. And the consequences of losing yield because of improper planting date selection have never been greater.

How many seeds per acre should be planted and when? Palle Pedersen, former Iowa State University Extension soybean specialist, conducted many planting date and rate studies in the mid-to-late 2000s.

He found that early planting consistently increased yield, especially when beans were planted in 15-inch rows. With early planting, Pedersen’s yields averaged 4.5-bushels-per-acre higher in 15-inch rows than in 30-inch rows.

His research showed if soybeans are planted during the last week of April in the southern two-thirds of Iowa or the first week of May in the northern third of Iowa, there’s a 79% chance the yield will be better than a May 20 planting date. Planting earlier than these dates, however, quickly increases the likelihood of needing to replant because of injury due to cold temperatures.

The 2010 experience with soybean sudden death syndrome has caused many farmers to consider planting later, when soils are warmer, to minimize chances of SDS infection. SDS did occur in some of Pedersen’s plots, and he noted there was less SDS in late-planted soybeans. While the later-planted beans appeared healthier, their yield was still less than the earlier plantings.

Plant soybeans on time

So, soybean planting should not be intentionally delayed because of the potential of SDS infection. Iowa had “the perfect storm” for SDS infection in 2010, and managing planting to avoid “the perfect storm” will reduce profits in the long run. If SDS is a concern, manage it by:

Planting high-yielding varieties with the least susceptibility to or greatest tolerance to SDS.

Ranking your fields in order of least problematic to most problematic, and then planting them in that order. Fields with the most problems will be planted when soils are a little warmer.

Relieving soil compaction, if present.

Improving drainage.

His research also showed that a uniform final stand of more than 100,000 plants per acre didn’t increase yield, but final stands of less than 100,000 plants per acre often resulted in significantly lower yields. This was true across row spacings (from drilled to 30-inch rows) and across planting dates.

How many seeds per acre do you need to plant to achieve a final stand of at least 100,000? Ideally, compare seeding rates in previous years to the final stand in previous years. If you’ve been planting 160,000 live seeds (total seed count adjusted for germination rate) per acre and have been achieving final stands of 130,000 — you have been losing 30,000 out of 160,000, or 18.75% of your stand.

Assuming this rate of loss continues, if you plant 125,000 live seeds per acre, you would expect a final stand of 101,500. If you don’t have previous live seeding rates and final stand counts available, remember to adjust seeding rates up from 100,000 to compensate for seeds that do not germinate (see the tag on the bag for germination rates), and for seed or plant mortality, which is affected by a number of things.

Those factors include the seed (plant high-quality seed), equipment adjustment, planting depth (1 to 1.5 inches is best), planting speed, soil type and drainage, and seedbed conditions (plant early but only when seedbed conditions are adequate). Mudding the crop in just to get it in early will cause more harm to yield than will be recovered by the early planting.

The same seeding rates can be used for tilled and no-till beans if the seeding equipment is properly weighted and adjusted, and soil conditions are acceptable. Pedersen found that planting 125,000 to 140,000 live seeds per acre planted with a row unit into a good seedbed usually results in a final stand just over 100,000 plants per acre.

Schmitt is the ISU Extension field crop specialist at Muscatine.

Higher rates, higher mortality

Seeding rate affects the final soybean stand as higher seeding rates have higher mortality rates. Row spacing also makes a difference — approximately 10% more of the live seeds planted are lost in 30-inch rows compared to 15-inch rows. Row units provide better and more consistent seed-to-soil contact than do drills.

Planting in late April has about 25% to 35% mortality, while early June has 10% to 15% mortality. Weather is a factor — temperatures, rainfall, crusting, etc. Diseases are, too, including seedling diseases, which may be reduced by using a seed treatment. A fungicide seed treatment increases the survival rate by about 8,000 plants per acre.

Insects can reduce stands, such as overwintering bean leaf beetles, which may be controlled by an insecticide seed treatment. A fungicide plus an insecticide seed treatment increases the plant survival rate by about 17,000 plants per acre. Weeds can be a stand reducer, too.

This article published in the March, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.