A wide variety of commercial biostimulant products are available for soybean seed application, with more products added annually. Most soybean producers are familiar with seed-applied inoculants (living bacteria such as Bradyrhizobium, fungi, etc.) that colonize the root and increase in numbers.

Biostimulants are derived from something that was living (therefore, bio) and are used to stimulate certain and/or overall plant growth processes such as larger roots, increased disease tolerance through activated responses, etc. The active portion of a biostimulant product could be a protein, hormone, or even a signaling molecule. Some commercial biostimulant products have as many as three hormone classes as active ingredients. Concentrations can differ greatly from product to product, especially in those products containing hormones such as gibberellic acid (GA), kinetin and indole butyric acid (IBA).

While most biostimulant products have only biostimulant properties, some are hybrid products (inoculant + biostimulant), such as the lipochitooligo (LCO) saccharide products Optimize and Trident.

Testing Biostimulants

Growers have questions on seed applied biostimulants effects for soybeans, with the main question being “Do seed applied biostimulant products provide a consistent economic return, and if so, under what conditions?”

To answer these questions, over the past five years numerous replicated field trials involving both small plot and large commercially harvested plots were conducted across eastern Nebraska under irrigated and rain-fed conditions. While numerous products were evaluated in one or two field trials, only BioForge (an antioxidant product for stress) and Optimize have a robust data set providing for comparison under rain-fed and irrigated conditions. Each site shown had four replicated treatments with the exception of the David City 2011 site with Pioneer 93M11 soybeans, which only had three replications.

The majority of rain-fed plots were commercially planted (and combine harvested, weighed in a weigh wagon, and standardized to moisture), but the opposite was true for irrigated soybeans (small plots, etc.).

Rain-Fed

While a consistent trend for slightly higher yields was noted at every rain-fed site (average of approximately 3.7%), no statistical increase in yield was not noted at any single location. A paired t-test over all sites using the location yield mean noted that use of these two biostimulants did result in a significant yield increase of 1.63 bushels/acre (50.3 versus 48.6 bu/acre, p<0.01).

Using the cash bid of $9.30/bushel for October 2016 delivered soybeans, and using the overall increase in bushels/acre of 1.63 bushels, this increased the value/acre in rain-fed soybeans to $15.15/acre. Per acre product cost sepends on amount of seed planted/acre (more seed = higher cost) and the seed treatment costs. Using a minimum of $4/acre for product and another $2/100 lbs. soybean seed for application, these treatments created a net average maximum increase value of $9.15/acre.

Rainfed Trials

Site	Variety	Year	Product	BioStimulant Yield (bu/acre)	Check Yield	Bu/Ac Difference Acre vs Check	% Change
Waverly	Pioneer 93M11	2011	BioForge	56.2	54.0	2.1	4.0
Waverly	Pioneer 93M11	2011	Optimize	55.9	54.0	1.9	3.4
David City	Asgrow 2909	2011	Optimize	46.4	46.1	0.3	0.7
David City	Mycogen 5B261RR	2011	Optimize	53.5	51.5	2.1	4.1
David City	Pioneer 93M11	2011	Optimize	42.1	40.0	2.1	5.2
David City	Pioneer 93M11	2011	BioForge	43.0	39.9	3.1	7.6
Concord*	Pioneer 93M11	2012	BioForge	30.8	29.9	0.9	3.0
David City	Pioneer 93M11	2013	Optimize	55.4	54.7	0.7	1.3
* Drought conditions this year, and small plots. All others were large plots and combine-harvested. A weigh wagon was used to obtain weights.			(All sites)	50.3 a	48.6 b	1.7	3.8

 Irrigated Trials

When examining data from irrigated soybean fields, a trend for increased yields associated with seed-applied biostimulants was not evident.  It should be noted that most of the irrigated data was derived using small plots and the variety was different than those utilized in the rain-fed fields.

Using a cash bid of $9.30/bushel for October 2016 delivered soybeans, and using the overall increase in bushels/acre of 0.93 bushels, this increased the value in irrigated soybeans to $8.93/acre. Using the same costs as in the rainfed fields ($6/acre for product and treatment), these treatments created a net average maximum increase of $2.93/acre.

Site	Variety	Year	Product	BioStimulant Yield (bu/acre)	Check Yield	Bushels/ Acre vs. Check	% of Check
Rising City*	NK30-D4 RR	2009	BioForge	70.3	69.6	0.7	1.0
Fullerton	Asgrow 2733	2014	BioForge	73.2	74.3	-1.1	-1.5
Shickley	Asgrow 2733	2014	BioForge	76.2	70.8	5.4	7.6
Auburn	Asgrow 2733	2014	BioForge	58.3	57.4	0.9	1.6
Fullerton	Asgrow 2733	2014	Optimize	73.3	74.3	-1.0	-1.4
Shickley	Asgrow 2733	2014	Optimize	73.6	70.8	2.8	4.0
Auburn	Asgrow 2733	2014	Optimize	56.2	57.4	-1.2	-2.0
Total				68.7a	67.8a	0.9	1.3
* Large plot, combine harvested. All others are small plots. P value = 0.35 No statistical difference

Summary

These data suggest that there is a difference between rainfed and irrigated fields for soybean yield responses to soybean seed-applied biostimulants. Data from other treatments not as robustly evaluated are also consistent with greater economic returns in rain-fed conditions; however, economics appear to differ by product. While the reason for the yield difference is not known, the trend for a numerical yield increase in rain-fed fields may be partially due to stress, as irrigated soybeans would have less moisture stress.

One important aspect to note is that varietal responses exist to the same biostimulant (as seen for Asgrow 2909 and Mycogen 5B261RR soybeans from the same David City field in 2011). As the varieties in irrigated vs. rain-fed conditions were not the same, this also may be affecting the currently noted trend.