Over the last few growing season, many Mid-South soybean producers have achieved outstanding yields. How best to continue the trend?

The Mosaic Company’s Ross Bender, who works in the Midwest, provided some ideas at the Louisiana Agricultural Consultants Association’s mid-February conference held in Marksville.  

Corn, soybeans and wheat are the three largest crops grown in the continental United States. “They account for about 225 to 250 million acres. Corn just set a new world record this year – David Hula, in Virginia, grew corn that yielded 532 bushels per acre. (Southwest Missouri’s) Kip Cullers holds the soybean yield record at 161 bushels per acre. A new wheat yield record was also set this year with a farmer in the United Kingdom at 245 bushels per acre.”

In 2015, the average U.S. yield for corn was 168 bushels per acre – down two bushels from a year earlier. For soybeans it was 48 bushels with wheat coming in at 43 bushels.

“I define ‘yield gap’ as the difference between potential yield and reality, what is actually achieved. Yield potential is often three times greater than what is harvested. That means the corn yield gap is 364 bushels, the soybean gap is 113 bushels and the wheat gap is 203 bushels.”

Soybeans were introduced to the United States in the late 1800s the crop was originally used as forage, a feed source. “They didn’t actually harvest the crop for beans; they harvested them for feed. (The government) published bulletins on how to cut soybeans and store them for optimum feed quality.”

It wasn’t until 1935 that the number of acres of soybean grown for seed exceeded the number for feed. “1924 was the first year the U.S. Department of Agriculture even started keeping track of soybean yields. Over the last 100 years, soybean yields have increased five-fold.

“Corn yields have increased tremendously, as well. When they began to keep track in the 1870s, corn yields were 20 to 30 bushels per acre. But there are distinct things in the life cycle of a corn plant that allowed the crop to increase yield rapidly. … In the 1950s, after the end of WWII, nitrogen stopped being used in munitions and became a fertilizer source.

“All along the way, we saw increases in yield due to equipment, mechanization, planting populations and other genetic factors. One thing stands out from the turn of the century that affects how we grow corn today: biotech.”

Currently, the average U.S. corn yield is about 170 bushels per acre. Soybean is at 48 bushels.

Bender pointed to six factors worth considering to help boost soybean yields. 

The first things for success for growing high-yield soybeans: water and weather. “Unfortunately, weather is the least controllable thing in the field. It is very important to provide water to the crop during seed filling. If there’s one time we need rain, or water, it’s in July.”

Fertility

The second most important factor is providing proper nutrition. Based on surveys, “the USDA says 80 percent of producers don’t provide phosphorous or potassium to their soybeans. They leave the crop to mine existing soil reserves. That number is quite shocking because the crop’s nutritional needs are rather high.”

Bender said a 60-bushel-per-acre soybean crop requires 245 pounds of nitrogen, 43 pounds of phosphorous, 170 pounds of potassium, 17 pounds of sulfur, 4.8 ounces of zinc, and 4.6 ounces of boron. Nutrients removed with grain production, meanwhile, shows nitrogen at 179 pounds, phosphorous at 35, potassium at 70, sulfur at 10, zinc at 2.0 ounces, and boron at 1.6 ounces.

“Harvest index is the percentage of total nutrients apportioned to the grain. If I have a higher number under harvest index, it means that nutrient is important to produce grain. The ones that stand out here are N (73 percent), Phosphorous (81 percent) and sulfur (61 percent).”

How do potassium and phosphate stack up when comparing soybean and corn? “Potash is often the most-focused on nutrient – 230-bushel corn needs 180 pounds per acre and 60-bushel soybeans need 170. The values for potassium: 101 pound per acre for corn and 43 pounds for soybeans. Meanwhile, 80 pounds of P2O5 is removed with the corn and 25 pounds is removed by soybeans.”

Soybean doesn’t need as much phosphate as corn – about half as much, said Bender. “In Illinois, the average producer applies about 90 pounds of P2O5 to their corn.”

How is potassium accumulated and used in the plant? Where is a majority of potassium in the plant through much of the growing season?

“The stems, petioles and leaves. I don’t know if it’s luxury consumption but I do know there is a tremendous demand that could be used and moved to somewhere else to produce grain.

“That’s different from other nutrients such as phosphate. The plant has a season-long demand for phosphorous accumulation. From 45 days after planting to 110 days after – roughly 70 days – the crop will take up about 1.5 pounds of MAP each day. At the end of the growing season, the majority of phosphorous is in the grain. … In the Midwest, phosphorous is a nutrient we’ve seen some pretty strong growth responses after applying just a bit.”

Another nutrient receiving attention lately is sulfur. Why is sulfur more important today than in the past? “Due to scrubbers and other things, there are lower emissions and levels of sulfur in the atmosphere. … From 40 to 50 days after planting through seed-filling, there’s a tremendous demand for sulfur.

“We need to make that nutrient sources are available all season long, especially on the back end. That’s tough to do because some of our fertilizer sources don’t allow that. During seed-filling, the temperatures are often the hottest and weather is driest for the whole year. That’s a challenge for making sulfur available.”

In 2015, University of Illinois researchers evaluated 17 soybean varieties. The varieties “were intensively managed to see if there was a difference in yields. The lowest yield was 69.5 bushels per acre and the highest was 89 bushels -- a 20-bushel yield swing. Even with $8 soybeans, that’s a $160 decision.”

What about the maturity groups of the varieties?

“Illinois is a long state. On the northern end, producers can grow a 2.5 soybean and on the southern end they can grow a 5.2 and get away with it.” Looking at the 17 varieties, yields rose as the maturity group number did the same.

Foliar protection/row spacing

“Foliar protection is one of the components we use to estimate soybean yield. … Out of pod number per acre, seed number per pod and weight per seed what is the Number One factor that influences yield? If you want to drive yield, add more pods per plant. That’s the way to do it and it’s hard.

“Most pods on a plant are in the middle portion. Sixty percent of the pods are in that section. What’s the difference between 50 bushels per acre and 62 bushels? Six pods. It turns out each pod is worth about two bushels per acre.”

“Narrow row spacing increases yield. Farther north, 30-inch rows are commonly used. Some use narrow rows by putting splitters on their corn planters and plant 15-inch rows. We like 30-inch rows – you can drive down the rows without running over bean plants, there’s a lot of air-flow through the canopy and we can band nutrients under the row before planting. With 15-inch rows, we can’t do all that. But we like 20-inch rows.

“In the north, in our geographies, we have long days and there’s a lot of light. However, our season is short so we want to take advantage of the light we do receive. We think narrower rows are an opportunity to increase yields.”

A 2015 study looked at 30-inch versus 20-inch rows and yield. In DeKalb, in northern Illinois, the 30-inch rows yielded 61.7 bushels and the 20-inch yielded 69.6 bushels. In centrally-located Champaign, the 30-inch rows yielded 84.7 bushels and the 20-inch yielded 93.2 bushels. In Harrisburg, in southern Illinois, the 30-inch rows yielded 77.5 bushels and the 20-inch yielded 80 bushels.

“In the southern latitudes, the narrower rows didn’t penalize yields but we didn’t always see a response.”

Bender provided the following conclusions: