Assessing Soybean Stands

Soybean emergence and growth have been painfully slow this year. The early stages of germination and growth are basically driven by a series of chemical reactions. The more heat, the faster the reactions happen. Cool temperatures on the other hand result in slow growth. Thin stands due to insect feeding, disease, and soil crusting are not hard to find this year although overall stands across the province are surprisingly good. Most fields are now between emergence (VE) and the second trifoliate stage (V2) with populations adequate for high yields. Very thin stands are a concern especially in fields hit with heavy rain after planting. See picture #1 and #2. (Soil crusting, Orangeville June 17/2009).

It's important to realize that normal seeding rates have a margin of safety built into the recommended rates. Even under normal conditions a 75-80% emergence rate is considered normal for soybeans and a 50% stand should not be replanted. So it's important to make a careful assessment before replanting. With adequate moisture and weed control thin stands will yield well.

Last year we conducted two trials comparing a seed drill to a planter unit with different seeding rates, with and without CruiserMaxx seed treatment. At one of the sites the Cruiser Maxx had a stand of 76% compared to the untreated seed of 69% (P = 0.054). In the 7.5" rows the Cruiser Maxx had a plant stand of 70% compared to the untreated which had a plant stand of 64% (P = 0.047). At the second site there was no difference in plant stands.

The impact of seeding rate (and plant stand) was highly significant (P < 0.0001) on yield, but none of the other factors (equipment, row width, or seed treatment) showed differences in yield in 2008. This reinforces the fact that soybeans yield well over a wide range of plant stands. Even at a plant stand of 75 000 plants/acre (seeding rate of 100 000 seeds/acre) the yield was 44 bu/ac. Figure #1 shows the yield response of the different seeding rates. (k-value = 0.0164). These trials are being continued in 2009 and 2010 to determine the optimal seeding rate with and without the use of CruiserMaxx using different seeding equipment.

Figure 1. Soybean Yield Response Curve to Seeding Rate

One of the quickest ways to determine a soybean stand is the "hula" hoop method. Take at least ten hoop counts at random.

The alternative method is to measure the length of 1/1000th of an acre, count the number of plants and multiply by 1000 to get the number of plants per acre.

Numerous studies and field experience have demonstrated that keeping an existing stand is often more profitable than replanting. On lighter soils, fields with a plant stand of more than 100,000 plants/acre should not be replanted, while heavier clay soils require more plants per acre for success -- a minimum of 110,000 plants/acre.

For more details on replanting or thickening thin stands please see the Agronomy Guide (Publication 811) or the article entitled "Thickening Thin Soybean Stands".

http://www.omafra.gov.on.ca/english/crops/field/news/croppest/2006/05cpo06a3.htm