click image to zoom I realize that some Iowa soils are wetter than 25 percent FC; so the second possibility may be overly pessimistic for those areas. Other common inputs for each site modeled are provided in Table 1. Factors that varied across locations such as soil textures are shown in Table 2.
Given the two soil moisture scenarios at planting, the model allows us to estimate the effects of changing plant populations, as well as other factors, on simulated yield. The model predicts corn growth based on temperature, solar radiation, and precipitation actually recorded in the automated weather database for each research and development farm. It does not account for factors like insect feeding, diseases, weed competition, nutrient deficiency, hail, wind damage, etc.
For this comparison, I used a single ‘generic’ hybrid at each location: NW, N, and NE locations: 2500 GDD (about 105 days RM); West Central, Central, SW, and SE locations, 2600 GDD (110 days). For the analysis in this article, I compared final plant populations of 27,000, 32,000 and 37,000 ppa at each location with each of the two soil moisture scenarios at planting.
Wet soils at planting
Probabilities for different plant population responses with different soil moisture conditions at planting are shown in Table 3. At Kanawha and Ames for example, if we start the season with a wet soil profile, responses in all the previous years (16 and 27 years of weather history in the database for Kanawha and Ames, respectively) would always favor planting 37,000 ppa over 32,000 ppa. Likewise, the lowest population, 27,000 ppa, always yielded less than the middle population, 32,000 ppa, when soils were wet at planting at both of these locations.
click image to zoom For another example, at Sutherland in NW Iowa, 37,000 ppa out-yielded 32,000 ppa in 23 of the 25 years, 92 percent, when soils were wet at planting. The lowest population, 27,000 ppa, increased yields over 32,000 ppa in only four of the 25 years, 16 percent. However, in 2012, simulated yields with 27,000 ppa were greater than those of 37,000 ppa by an estimated 6 bu/acre (noted by the asterisks in Table 3). In most cases, if soils are wet at planting, staying with higher seeding rates would improve yields at all locations.
Very dry soils at planting
If we start the growing season with very dry soil profiles, optimism for high yields plummets along with probabilities of yield increases with high plant populations (Table 3). To illustrate this, look at the responses at Castana in WC Iowa. If we start with a very dry soil at planting, yields with 37,000 ppa are greater than those of 32,000 ppa just over half the time, 56 percent. Likewise, 32,000 ppa outperforms 27,000 ppa 60 percent of the time. In a year like 2012, 27,000 ppa would out-yield corn at 37,000 ppa by an estimated 9 bu/acre. That was true at Kanawha, Nashua and Ames as well. If soils are dry at planting, higher plant populations may reduce yields compared to lower plant populations.