Soybean maturity and “ready to harvest” are generally not the same thing. Physiologically speaking, soybeans are mature when seed dry matter accumulation is complete. But knowing when this happen is not as clear cut as corn, which forms a distinct black layer.
Maturity generally occurs when soybean pods have reached their mature gray, tan or brown color. At this point soybean yield is not negatively impacted by drought, frost or a herbicide application that desiccates the plant. It should also be noted that not every pod on a soybean plant matures at the same time. You can have mature pods and green pods on the same plant. Even when the last pod has matured plants are generally still too tough (wet and green) to harvest and the grain moisture would be too high to be acceptable.
When soybeans mature during periods of hot dry weather growers are often frustrated because the grain can dry much faster than the rest of the plant. If you have very good drying conditions it is not uncommon to have grain moisture that is lower than you would like on a plant that is almost too green, wet and tough to harvest. When this is the case waiting for a rain to put some moisture back into the grain and giving the plant a few more days to dry/die can improve yield, harvestability and save on fuel consumption and wear and tear on equipment. We have all had the frustrating experience of fighting through tough beans that are testing under 10% moisture. No one likes that grinding sound!
Tough, green stems at harvest can be a real frustration for soybean producers. Soybean breeders evaluate plots for “green stem” at harvest to try to weed out varieties that tend to have green stems more than others. However, selecting for plant health and disease resistance can result in healthier and greener stems at harvest. If soybean stems are dead and dry at harvest that can be an indication they were killed prematurely by diseases, which generally reduces yield. Fungicide applications that improve late season plant health and help prevent premature death can make stems greener and tougher as well. Plants infected with Pod Mottle Virus (PMV) will have prolonged green stems as well and in severe cases will produce very little seed and not mature normally at all.
The ideal market moisture for soybeans is 13%. If you harvest at grain moisture above 13% you will need to dry your grain in on-farm storage or pay for drying/dock when you deliver your grain to the elevator. If you have on-farm storage with a large enough fan, harvesting grain at moistures higher than 13% can be a good way to maximize yield and minimize harvest losses. Often this is hard to do because when the grain is over 13% moisture the plant may be too wet, tough and green to get through your harvesting equipment.
Harvesting soybeans when they are too dry is a major source of yield loss, which no one likes to see. I have seen soybeans get below 8% grain moisture in the field. This occurs when weather conditions are windy, warm and dry and harvest is delayed. This problem can be avoided by harvesting timely and planting a wide range of maturities so all your soybean acres are not ready to harvest at the same time. Waiting for a rain to put moisture back into the grain is also an option, but that is not without risks as well.
The problem with harvesting soybeans when they are too dry is yield loss. In 60-bushel soybeans the water weight loss for each 1% drop in grain moisture below 13% equates to about 0.7bu. So based on water weight loss alone (not counting additional shatter/harvest losses) soybeans that were yielding 60 bu/acre at 13% grain moisture will only yield 56.7bu/acre at 8% grain moisture.
In addition to the water weight loss shattering losses also increase significantly at lower harvest moisture. It is not hard to lose another 3 – 4 bu/acre in harvest losses depending on conditions and the type of harvesting equipment you have. Soybeans yielding 60 bu/acre at 13% moisture might yield only 53 bu/acre at 8% grain moisture by the time you account for yield losses. Practically speaking, it would be impossible to harvest all your soybeans at or above 13% moisture, but getting as many as you can harvested at or near 13% moisture will improve your yield and profitability.
Lance Tarochione is a technical agronomist with Asgrow/DEKALB in west central Illinois. His work has focused on crop production, research and product development, and through his role at Monsanto he currently supports the Asgrow and DEKALB brands in seven counties in western Illinois.