Soybean inoculants and growth promoters or enhancers studied

Soybean growers routinely mix a Rhizobium inoculant with soybeans at planting. Dual inoculant/growth promoting products have been added to the mix of choices available to soybean growers.

MSU Extension field crops educators in the Thumb and Saginaw Valley studied the use of soybean inoculants and growth promoters in 2008 and 2010. Because the products studied and protocols differed a bit, each study will be presented separately.

In 2008, one of the tested products was Optimize, which is listed by its manufacturer as being both a Rhizobium-promoting inoculant and a natural growth enhancer. Optimize was compared to Hi Stick Rhizobium inoculant and a non-treated check. A fourth treatment included Hi Stick and Optimize mixed together. The studies were repeated at three sites in the Thumb, and were replicated three times at each site. The trial sites had soybeans planted within the past three years, so the sites would be expected to be populated with Rhizobium bacteria at the time of planting. See Table 1 for results.

Table 1. 2008 Soybean Inoculant Study
Treatment Yield (Bu./A)
Control (untreated) 61.6
Hi Stick 61.7
Optimize 61.7
Both Mixed Together 62.2
CV: 2.9
LSD @ 0.05: 2.2

In 2010, Primo was added into the study. Similar to Optimize, Primo is listed by its manufacturer as a Rhizobium inoculant and a growth promoter. As in 2008, these two products were compared to Hi Stick and an untreated control. In 2010, a mixed treatment was left out. The studies were repeated at two sited in the Thumb, and were replicated four times at each site. Each site had soybeans grown in the recent past. See Table 2 for results.

Table 2. 2010 Soybean Inoculant Study
Treatment Yield (Bu./A)
Control (untreated) 48.4
Hi Stick 47.8
Optimize 46.7
Primo 48.5
CV: 5.8
LSD @ 0.05: 2.9

Results from both years indicate that neither Hi Stick nor either of the growth promoter/enhancers increased yields. However, it is still a good idea to use an inoculant when planting soybeans, even if soys have been planted in a field within the past three years. The risk of poor nodule development is too great given the low expense using an inoculant represents.

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