Simulations of 2021 end-of-season corn yield potential and real-time crop stage were performed on Aug. 4 for 39 locations across the U.S. Corn Belt using the UNL Hybrid-Maize crop model in collaboration with faculty and extension educators from 10 universities. This article summarizes the simulated crop stages and yield forecasts; the data can be viewed in Table 1. Details on the UNL Hybrid-Maize crop model and the underpinning methodology to simulate phenology and forecast end-of-season yields, as well as on interpretation and uses of yield forecasts, are described in a previous article. Note that one location in IL (Olney) was not included for the forecasts due to lack of weather data.

Although past weeks have alternate high and low temperatures, on average, air temperature over the past three weeks has remained near the historical average in most of the Corn Belt, with a few sites showing values below (KS) or above (ND and northern MN) the historical average. In the case of rainfall, most locations in the eastern fringe of the Corn Belt exhibited near-normal rainfall, while most sites in NE, MN and ND and some sites in IA and KS presented below normal rainfall. A summary of weather conditions during the last three weeks is shown in Figure 1.

Figure 1. Daily solar radiation, maximum and minimum air temperature (Tmax and Tmin), total rainfall, and total reference grass-based evapotranspiration (ET) for the time period between July 14 and Aug. 3, 2021. Vertical bars indicate the range for these variables based on 20-plus years of weather records. The horizontal thick line indicates the long-term average and the red dots indicate the 2021 values for the same period. Table 1. Data from simulations of 2021 end-of-season corn yield potential and real-time crop stage performed on Aug. 3.

	Location	Water regime	Long-term average yield (bu/ac) §	Range of Yp forecasts as of Aug 03 (bu/ac)¶ 25th	Range of Yp forecasts as of Aug 03 (bu/ac)¶ 75th	Probability (%) of 2021 yield to be: Below (relative to the long-term Yp)†	Probability (%) of 2021 yield to be: Near (relative to the long-term Yp)†	Probability (%) of 2021 yield to be: Above (relative to the long-term Yp)†	Simulated current crop stage*
NE	Alliance	Irrigated	193	190	215	0%	69%	31%	R3, Milk
	Beatrice	Dryland	162	140	175	39%	45%	16%	R4, Dough
		Irrigated	230	217	233	16%	84%	0%	R4, Dough
	Clay Center	Dryland	160	47	110	95%	5%	0%	R3, Milk
		Irrigated	245	229	253	13%	82%	5%	R3, Milk
	Concord	Dryland	177	89	117	100%	0%	0%	R2, Blister
		Irrigated	249	233	269	8%	77%	15%	R3, Milk
	Elgin	Irrigated	252	239	271	6%	70%	24%	R2, Blister
	Holdrege	Dryland	115	75	135	42%	21%	36%	R3, Milk
		Irrigated	242	225	264	15%	64%	21%	R2, Blister
	McCook	Dryland	88	59	92	51%	31%	18%	R4, Dough
		Irrigated	225	201	225	31%	67%	3%	R4, Dough
	Mead	Dryland	183	109	164	74%	21%	5%	R3, Milk
		Irrigated	235	218	239	18%	74%	8%	R3, Milk
	North Platte	Dryland	91	48	95	64%	18%	18%	R3, Milk
		Irrigated	234	229	264	3%	59%	38%	R3, Milk
	O'Neill	Irrigated	227	207	234	22%	67%	11%	R3, Milk
IA	Ames	Dryland	231	172	188	94%	6%	0%	R4, Dough
	Crawfordsville	Dryland	230	215	235	13%	81%	6%	R3, Milk
	Lewis	Dryland	209	208	239	9%	52%	39%	R3, Milk
	Nashua	Dryland	229	181	198	91%	6%	3%	R3, Milk
	Sutherland	Dryland	211	192	208	25%	66%	9%	R3, Milk
IL	Bondville	Dryland	235	236	268	0%	59%	41%	R3, Milk
	Freeport	Dryland	213	175	203	45%	42%	13%	R3, Milk
	Olney	Dryland	189
	Peoria	Dryland	210	219	244	0%	64%	36%	R3, Milk
	Springfield	Dryland	178	217	229	0%	5%	95%	R4, Dough
IN	Butlerville	Dryland	225	207	233	22%	78%	0%	R3, Milk
	Columbia City	Dryland	224	222	238	0%	83%	17%	R2, Blister
	Davis	Dryland	231	233	255	0%	72%	28%	R2, Blister
	West Lafayette	Dryland	238	230	256	0%	83%	17%	R3, Milk
KS	Garden City	Irrigated	219	199	221	20%	77%	3%	R4, Dough
	Hutchinson	Dryland	100	102	113	0%	68%	32%	R5, Dent
	Manhattan	Dryland	135	159	174	0%	6%	94%	R4, Dough
	Scandia	Dryland	132	41	78	100%	0%	0%	R3, Milk
		Irrigated	225	215	249	9%	66%	26%	R3, Milk
	Silverlake	Dryland	140	153	166	0%	34%	66%	R4, Dough
		Irrigated	210	192	212	17%	83%	0%	R4, Dough
MI	Ceresco	Dryland	177	212	249	0%	10%	90%	R2, Blister
MN	Eldred	Dryland	115	8	9	100%	0%	0%	R3, Milk
	Lamberton	Dryland	215	155	170	100%	0%	0%	R3, Milk
	Waseca	Dryland	219	190	232	32%	54%	15%	R3, Milk
MO	Brunswick	Dryland	181	182	201	0%	71%	29%	R3, Milk
	Monroe City	Dryland	172	204	226	0%	5%	95%	R3, Milk
	St Joseph	Dryland	165	184	207	5%	14%	81%	R3, Milk
ND	Dazey	Dryland	113	0	0	100%	0%	0%	R3, Milk
OH	Custar	Dryland	208	198	222	11%	72%	17%	R2, Blister
	South Charleston	Dryland	216	205	234	6%	77%	16%	R2, Blister
	Wooster	Dryland	210	209	229	6%	72%	22%	R2, Blister

§Long-term (last 20+ years) potential yield at each location and surrounding area.
¶ Range of forecasted 2021 potential yields based on average planting date in 2021, indicating the potential yields in the 25th and 75th percentile of the potential yield distribution (associated with respective adverse and favorable weather scenarios during the rest of the season).
† Probability of obtaining a 2021 yield below (10%) than the long-term potential yield at each location.

Simulated Corn Stage Across 40 Locations

Corn has reached kernel milk stage in almost the entire Corn Belt, except for the eastern fringe of the region (OH, MI and IN) where it is still in blister stage. Some sites in the southern fringe of the region (KS and southern NE and IL) are ahead of the rest of the locations, with corn reaching the dough and even dent stage (Figure 2). Most locations are similar to last year’s corn development by Aug. 4.

Figure 2. Simulated developmental stage for irrigated and rainfed corn at each location. R2: blister; R3: milk; R4: dough; R5: dent. Separate maps are shown for irrigated corn (top) and rainfed corn (bottom).

Irrigated Corn: High Probability of Near-average Yields

The range of forecasted irrigated corn yield potential for each location, as well as the probabilities for yields above, near or below average, are shown in Figures 3 and 4. Six out of 13 sites exhibit a high probability (>75%, that is, a chance of three out of four) of near-average yield potential. Favorable weather during the rest of the season that results in a long grain-filling period may increase the likelihood of above-average yield. The chance of below-average yield is low across all irrigated sites. At this point, the forecasted scenario for irrigated maize in the current season seems similar to the 2020 forecasts in nine out of 13 sites.

Variable 2021 Forecasted Corn Yield Across Rainfed Locations

Forecasted yield potential is highly variable across the 35 rainfed sites (Figures 3 and 4). There is a high probability of above-average yield (>75%) at five sites in the southern fringe of the Corn Belt and MI, while nine sites distributed across the central (IA), western (NE and northern KS), and northern (ND and MN) regions have a high probability of below-average yields. Very low yields are predicted at the uttermost northern sites (Eldred, MN and Dazey, ND). Probability of near-average yield is relatively high in the central and eastern part of the Corn Belt (most of IL, IN and OH).

Compared with our previous forecast, below-average rainfall in most of NE, ND, MN and two sites in IA during the past three weeks increased the probability of below-average yields in that area, reducing the forecasted yield for the 2021 season. In contrast, above-average rainfall in a few scattered locations (Manhattan, KS, Ceresco, MI and South Charleston, OH) has improved the yield forecasted for rainfed maize in those sites by mid-July. Compared with the 2020 forecast, the forecasted scenario for rainfed maize seems more pessimistic in ND, MN, northwestern NE and central IA in the current season. In the remaining area, the scenario looks similar or even more favorable compared with 2020 forecasts.

Figure 3. Vertical lines indicate the range of forecasted 2021 corn yield potential by Aug. 4 based on average planting date in 2021 at each location. Horizontal lines indicate the 25th and 75th percentiles of the yield distribution (associated with respective adverse and favorable weather scenarios during the rest of the season). The blue squares indicate the long-term (2005-2020) average yield potential at each location. Separate charts are shown for irrigated corn (top) and rainfed corn (bottom).

Yields in Dazey, ND and Eldred, MN cannot be accurately forecasted due to very low rainfall.

Figure 4. Probability of the 2021 yield potential to be below (10%, green color) the long-term (2005-2020) average yield potential at each location. Separate maps are shown for irrigated corn (top) and rainfed corn (bottom). The larger a color section is within the pie chart, the higher the probability that end-of-season corn yield will be in that category.

Conclusions

Similarly to our previous forecast in mid-July, the probability for a “record yield year” for the whole country is low considering that the probability for above average yield is small (

For rainfed corn, the scenario is diverse across regions. Most sites in the central and eastern part of the Corn Belt have a high probability of near-average yields. Above-average yield is expected at five locations mostly located in the southern fringe of the Corn Belt and MI. In contrast, nine sites located in the western and northern areas of the Corn Belt and central IA exhibit a high probability of below-average yields. Temperature and rainfall during August will likely define the trend for all sites across the region.

These forecasts do not take into consideration problems with stand emergence, hail/flooding damage, replanting situations, disease or nitrate leaching. In fields negatively affected by these constraints, actual yields will be lower than estimates provided here. It is important to keep in mind that yield forecasts are not field specific and, instead, represent an estimate of average on-farm yield for a given location and surrounding area in absence of the yield-reducing factors mentioned here. Likewise, crop stages and forecasted yields will deviate from the ones reported here in fields with planting dates or hybrid maturities that differ markedly from those used as the basis for the forecasts (see Table 1 in this July 14 article). We will follow up with further forecasts in late August.