November 2007 Newsletter Volume 26:9

Soybean Plant Population

How much soybean seed do I need? That depends partially on how efficiently soybean seeds result in plants. The first question to ask is how many soybean plants per acre are needed? With the increase in seed cost, more attention is being paid to getting the right amount of seed for next season. Not enough seed will obviously hurt yield and possibly weed control, while too much seed will hurt the bottom line in profits. To answer that question, data from population studies for soybeans from U of I research centers has been pulled together and analyzed.

The data for this analysis is from 15 data sets from soybean population studies conducted at 3 U of I research centers in northern Illinois, DeKalb, Monmouth and Urbana. The studies were conducted from 1998 through 2007, with multiple row widths, varieties and tillage systems for a total of 498 data points. This data was selected because data on plant populations of soybeans at harvest were collected. A regression analysis was preformed to determine the yield response in relation to the number of plants per acre. The plant populations of this data ranged from 31,000 to 235,000, and the yields ranged from 35 to 78 bu per acre.

The goal of this exercise was to determine the optimum number of plants at different prices for soybean. The definition of optimum is when the last seed added pays for itself with increased yield. The optimum can vary depending on the cost of seed and the price soybeans can be sold for.

In conducting this analysis, a couple of assumptions were made. First, that 90% of the seeds would develop into harvestable plants, and second, that seed cost would be $31.50 per unit of 150,000 seeds. These assumptions result in seed cost of $0.23/1000 seeds.

Figure 1. Effect of soybean plant population on yield in northern Illinois (DeKalb, Monmouth, Urbana, 1998-2007).

The resulting yield response curve to soybean plant population is described by the following equation: yield (bu/acre) = 45.113 + 0.14566x - 0.0005798x2, where x = number of 1000 plants (Fig. 1). The average yield response across all plant populations was fairly flat, ranging from 51 to 54 bu/acre. The highest yield was with 125,000 plants at 54.3 bu/acre. The optimum number of plants varied from 100,000 to 109,000 plants per acre (Table 1).

So how does a farmer use this information? Basically, they can select a plant population based on seed cost and soybean price, making adjustments for seed cost if necessary, then determine how to get that number of plants in the field. Previous work at the U of I research centers has shown that seeding efficiency can vary between equipment and tillage system any given year. In tilled soil the seeding efficiency with the drill averaged 71% compared to 79% with row units. The average stand efficiencies were higher in no-till, with the drill at 75% and the row units at 85%. However, the efficiency can be greatly influenced by the conditions. For example, the lowest stand efficiency for the drill and row units were 55 and 68%, respectively, in tilled soil in a crusting situation one year. The highest stand efficiency measured has been in no-till with 15" rows, averaging 92% the last two years at NWRC.

Previously, it had been thought that having at least 100,000 plants/acre would be an adequate stand if they were evenly spaced. These results confirm that 100,000 plants/acre was not far off. The main question is how many seeds need to be planted to make sure that there are enough plants. Farmers will know how their equipment will perform in different soil types and conditions, and can adjust the seeding rate depending on their comfort level. Generally, farmers have been planting more than enough soybeans to have adequate plants to reach the yield potential we encounter in northern Illinois. Often there are factors limiting yield other than plant population. Diseases, insects and weeds can be the yield limiters many years, while environmental factors can be limiting other years.

Will cutting back on plant population limit yield potential? There have been reports of very high yields with very high populations of soybeans, but these data show that yields tend to decrease at the higher populations. This is probably due to dry conditions that are limiting yields, and soybeans at high populations may not have as extensive root system or will use up moisture reserves quicker than lower plant populations. This data shows that 125,000 plants per acre will be adequate to maximize yield, the optimum being somewhat lower, with the current varieties and yield limiting factors encountered in northern Illinois. Soybeans have outstanding ability to ‘flex’ with the conditions they encounter. For example, up to 30% of the yield can be determined by seed size alone, which is determined by conditions in late August-early September.

Farmers can probably save some money in soybean seed cost based on these data, or at least have some facts on which to base their decisions. This is in contrast with the corn population results (February 2007 newsletter) that showed that the optimum corn population is higher than what most farmers are planting.