UNL Hybrid Maize modeling software shows

Impact of late planting on corn yields and yield stability

One way to examine this question is to conduct a large number of field studies at different locations across the state over many years to ensure that the results are valid across the normal variation in climate. Such studies would need to employ a wide range of corn hybrids of different maturities — again to cover the full range of interactions between hybrid and year. Unfortunately such work takes a long time and is very costly. In addition, even with four or five years of data, it is not likely that the full range of climate variation will occur during the study period so the results may not be accurate for the normal range of year-to-year variability in weather conditions.

Figure 1. Simulated impact on corn yield and yield stability from late planting at six sites in Nebraska relative to a May 1 planting date. Each simulation is based on more than 20 years of climate data at each site.  Hybrid maturity and yield potential for the May 1 planting date are provided in Table 1.

Figure 2. Simulated probability of early frost before crop maturity for planting from May 1 to June 1 at six sites in Nebraka.  Hybrid maturity at each site is given in Table 1.  Simulations are based on more than 20 years of climate data at each site.

Table 1. Hybrid maturity and the yield potential of a May 1 planting date at six sites in Nebraska as simulated by the HybridMaize model. Each site has at least 20 years of climate data. Plant population was set at 32,000/acre for irrigated and 28,000/acre for rainfed conditions, and initial soil moisture status was 75% of field capacity. Site GDD RM, days Yield potential, bu/ac Irrigated Rainfed North Platte 2500 103 228 133 Holdrege 2600 108 234 160 Clay Center 2650 110 246 183 Beatrice 2750 115 250 195 Mead 2700 113 242 186 West Point 2600 108 243 186

Computer-based crop simulation models provide another tool for assessing the impact of late planting on yield and yield stability using long-term historical climate databases that are available for much of Nebraska. The Hybrid Maize Model (www.HybridMaize.unl.edu) is such a tool because it allows simulation of corn yields at different planting dates and with different hybrid maturities using the actual historical climate data for a given site. For example, the climate database for North Platte includes daily solar radiation, maximum/minimum temperatures, and rainfall from January 1982 to present. These data can be used to simulate corn yields obtained from different planting dates for each year in that dataset using the model.

The results show that for a hybrid with 103 days relative maturity (RM, equivalent to a requirement for about 2500 growing degree days) planted on May 1 with a plant population of 32,000 per acre, the average yield potential is 228 bu/ac with optimal irrigation and 133 bu/ac under rainfed conditions with initial soil moisture at 75% of field capacity. Likewise, the yield variability is about ±12% of the average for irrigated conditions (which is relatively stable), and about ±37% of the average for rainfed conditions (which is relatively unstable due to large year-to-year variation in rainfall amounts and patterns). Some of the yield variability at North Platte is due to early frost, which the model estimates as occurring in about 20% of the years from 1982-2005.

By simulating corn yields for each planting date from May 1 to June 1 in 5-day intervals, it is possible to evaluate the predicted impact of late planting on corn yields and yield stability. Again, using a hybrid of about 103 days RM and a plant population of 32,000 per acre and soil moisture at 75% of field capacity at planting, the model predicts that yield potential is relatively constant over this period for both irrigated and rainfed conditions (Fig. 1a,b). However, yield variability increases from ±12% to ±21% with later planting under irrigated conditions and remains relatively constant, but very high, at ±37% for rainfed conditions (Fig 1c,d). Increased yield variability with later planting under irrigated conditions results from increased risk of frost damage before the crop reaches maturity (Fig. 2). In contrast, yield variability does not increase with later planting under rainfed conditions because yields are mostly influenced by the severity of water deficit that occurs as rainfall typically cuts out in the second half of August and September during the grainfilling stage. Under these conditions, a late frost has little impact because the rainfed crop has already run out of water.

Performing similar simulations for five other sites across Nebraska’s corn-growing regions shows similar trends (Figs 1a,b,c,d and Fig 2). For each site, a corn hybrid with a maturity typical for that part of the state was selected (Table 1). When planted on May 1 with 32,000 plants per acre under irrigated conditions and 28,000 plants per acre rainfed, the long-term average yield potential ranged from 228 to 250 bu/ac with irrigation and from 133 to 195 bu/ac rainfed (Table 1).

Little yield penalty

Based on these simulations, it appears that there is little yield penalty on average for late planting of irrigated corn between May 1 and June 1 at the six locations across Nebraska (Fig. 1a). However, later planting increases year-to-year variability in yields due to greater risk of late season frost on irrigated corn.

For rainfed corn, there is no yield penalty predicted for late planting at three of the five sites, but a 5% reduction at two sites (Fig 1b) and a much higher degree of yield variability at all sites compared with irrigated production at the same location.

It should be noted that the model simulates yield potential, which for irrigated conditions is the yield that can be obtained when irrigation eliminates all water stress and there is no yield loss from nutrient deficiencies or other soil constraints, diseases, weeds, and insect pests. For rainfed systems, the model simulates the water-limited yield potential, which is the yield that can be achieved with the rainfall amount and distribution in the year of simulation. Actual yields achieved under on-farm conditions fall below the yield potential ceiling and the gap between actual yields and the simulated yield potential is determined by the degree to which crop and soil management eliminates abiotic and biotic stresses other than temperature under irrigated conditions, or temperature and rainfall under rainfed conditions. However, the simulated yield potential provides a reasonable indication of how actual yields would be affected by planting date assuming a similar crop and soil management practices are followed each year.

Other factors to consider

While the good news is that the model does not predict a significant decrease in yield potential due to later planting between May 1 and June 1 for most sites in Nebraska, there are likely to be reasons other than solar radiation, temperature, and rainfall patterns that cause yield loss and greater yield variability from later planting. For example, there is greater chance for several days of hot, dry, windy weather in the second half of May and early June that can rapidly dry the seed zone and cause seedling stress. If such a hot spell occurs after a severe thundershower, a soil crust can form that suppresses seed emergence and causes uneven stands and less than optimal plant density. Likewise, later planting often makes weed control more difficult.

Finally, higher than normal losses of pre-plant N fertilizer to denitrification and leaching may occur because of the rainy weather that caused the delay in planting. When this occurs, yields can be reduced by N deficiency unless crop N status is monitored and additional N is applied as required. Therefore, later planting is not recommended unless wet soil conditions preclude planting at the optimal time. However, this analysis does suggest that late planted yields can be quite respectable if soil conditions and crop management allow for good stand establishment, N fertilizer efficiency, and weed control.

Like all simulation models, predictions from Hybrid Maize are not perfect and must be used in combination with common sense and experience; however, the model provides another tool for evaluating the impact of late planting on crop yields and yield stability when dealing with conditions that make it difficult to plant on time.