Organic soybean producers consistently report that weed management is their biggest obstacle to higher yield and an improved bottom line. One of the most effective weed control tactics we have investigated has been the cultural practice of increasing the number of soybean seed per acre. In the weedy conditions of an organic soybean field, increasing the soybean seeding rate from 75,000 seed per acre up to 225,000 seed per acre can improve weed control by as much as fifty percent. However, when evaluating a weed control tactic, weed management is not the only consideration.
Conventional (this pun is intended!) wisdom in soybean production is to reduce the seeding rate to improve economic return. This paradigm is based on the expense of genetically modified soybean seed (with the technology fee, a bag of GMO soybean seed may cost $50 for a 50 lb bag), the effectiveness of inexpensive herbicides, and the lower market price for conventional soybean. Current soybean seeding rate recommendations for conventional growers is about 100,000 seed per acre to achieve maximum yield. For conventional growers, increasing the seeding rate is throwing money away.
In contrast, organic growers: can purchase seed at half the cost, contend with serious weed populations, and enjoy a higher price premium. Because of these circumstances, the soybean seeding rate paradigm shifts dramatically for organic soybean producers. Increasing the seeding rate from 100,000 to 250,000 seed per acre may cost the grower an additional $28, but if yield improves by just 1.5 bushels due to better weed control, they’ll break even. If the grower is suffering from serious weed infestations, it’s a reasonably safe bet that yield will improve by more than just two bushels. Research at NCSU showed that in four trials comparing the soybean seeding rates of 75,000; 125,000; 175,000, and 225,000 seed per acre, the maximum economic return (based on a market price of $15 per bushel) was always achieved with the highest seeding rate. Furthermore, the magnitude of this return was related to the severity of the weed infestation. The four trials with weed infestation levels of very light, moderate, severe, and very severe resulted in $11, $172, $259, and $279 more profit, respectively, when comparing the highest seeding rate to the lowest soybean seeding rate. The bottom line . . . is about your bottom line; organic soybean producers struggling to improve weed control should consider increasing seeding rates.
One caveat with the increased soybean seeding rate is the possibility of increased soybean lodging. However, in the four trials conducted at NCSU, increased soybean lodging was not seen in the higher seeding rate treatments. Thus, while conventional soybean producers considering seeding rates may be asking themselves, “how low can we go”, the organic soybean producers must ask “how high can we stand”. For more information on this study go to:http://www.organicgrains.ncsu.edu/default.htm and click on ‘New Resources available from NCSU” to find the extension publication entitled “Planting Rate Recommendations for Organic Soybean Producers”.
