Emergence Uniformity in Corn

The planting operation is one of the most critical factors in maximizing corn yield potential. The goal at planting is to achieve a “picket-fence” stand – equally spaced plants, all emerging at the same time, that will ultimately all produce uniformly sized ears.

Numerous research studies over the years have examined the effects of plant emergence and spacing uniformity on corn yield. Of the two, uniform emergence has generally been found to be the more important factor in affecting yield (Liu et al., 2004; Doerge et al. 2015). Plants that emerge later than their neighbors are at a disadvantage in size and competitiveness and may produce smaller ears. If there are enough late-emerging plants, it can drag down the overall yield of the field.

The importance of uniform plant emergence has long been understood, and it has remained an area of focus for corn growers and agronomists seeking to optimize every aspect of the planting operation to maximize corn yield potential. As corn yield levels continue to increase, driven in large part by greater plant densities, it raises the question of what degree of emergence uniformity is necessary in order to maximize corn yield in high-productivity environments.

Uniform emergence is important in corn because of the highly competitive environment among plants in modern corn fields for access to resources, particularly sunlight (Satorre and Maddonni, 2018), and the relatively low vegetative and reproductive plasticity of modern corn hybrids (Rotili et al., 2021).

In contrast to other grass crops, where a single plant may produce multiple shoots and seed heads, selection for greater yield in corn has favored a compact, single-stalk phenotype that is tolerant to crowding stress and able to consistently produce a single well-sized ear in high density environments. Corn plants may produce multiple tillers and more than one ear per plant in very low-density environments but have relatively limited capacity to convert a marginal advantage in resource availability into increased yield.

Limited vegetative and reproductive plasticity is why uniform emergence is so much more important in corn than it is in other crops such as soybeans (Andrade and Abbate, 2005). Compared to corn, soybean plants have considerable ability to adapt to their surroundings. Plants adjacent to gaps in the stand can respond by producing branches and leaves that will capture the available sunlight and result in additional pod formation and yield. Some degree of plant attrition is normal in a soybean field, and yield response of soybeans to plant density is relatively low. As long as the stand is able to attain full canopy coverage – capturing all available sunlight – the density, size, and spacing of individual plants comprising the stand is of lesser importance.

Corn, on the other hand, does not have the same degree of plasticity. Plants adjacent to a skip or a smaller plant in the row have some capacity to capitalize on their relative advantage in available resources through increased yield, but not enough to compensate for the lost yield from the missing or low-yielding plant (Liu et al., 2004; Doerge et al., 2015; Novak and Ransom, 2018). Unevenness in plant size is generally detrimental to the overall yield of a field (Figure 2) and emergence timing is an important determinant of relative plant size (Nafziger et al., 1991; Ford and Hicks, 1992; Carter et al., 2001; Liu et al., 2004; Novak and Ransom, 2018).

Research on emergence uniformity of corn has shown that the impact of late emergence on individual plant yield can be substantial. A plant that emerges well after its neighbors faces a competitive disadvantage from which it will be unable to recover. Competition among corn plants for resources starts early in the season, at around the V4 stage, and intensifies during the rapid vegetative growth phase from V7 to V13 (Maddonni and Otegui, 2004). The difference in growth between advantaged and disadvantaged plants within the stand becomes larger as competition for resources increases. When the plants reach reproductive growth, smaller plants within the stand may produce a significantly smaller ear or no ear at all.

Several field studies have documented significant yield loss when the development of plants within the stand was delayed (Nafziger et al., 1991; Ford and Hicks, 1992; Liu et al., 2004). These studies typically used multiple planting dates and achieve varying degrees of delayed plant growth, with additional seeds planted into the row a specific number of days after the initial planting. Liu et al. (2004) found that individual plant yield was reduced by an average of 35% for plants that emerged 12 days late and 72% for plants that emerged 21 days late.

The length of planting and emergence delays tested in studies in the 1980s and 1990s was often relatively large, ranging from 7 days to over 21 days, as the primary purpose of the studies was often to help inform replant decision making – determining at what point the predicted yield loss associated with uneven emergence was sufficient large to justify starting over and replanting the field.

Current interest in emergence uniformity in corn is primarily oriented around optimizing the planting operation to achieve the smallest emergence window possible and maximize yield potential. Corn growers are interested in the yield outcomes of plants emerging as little as 1 to 3 days after the first day of emergence, a much shorter window than was generally tested in older field studies.

As corn growers strive to optimize every part of their corn production system for greater productivity, it is important to understand what level of emergence uniformity is necessary for maximum yield potential or even attainable under modern corn production systems. Working to optimize the planting operation for uniform plant emergence is generally going to be favorable for maximizing yield potential but it must be considered within the context of other management factors that can also affect yield, as there may be tradeoffs involved. For example, the most straightforward way to get the shortest emergence window possible would be to plant relatively late in the spring into a clean-tilled field. This would offer the greatest opportunity to plant seeds into a warm, uniform seedbed; however, the negative effects of later planting and intensive tillage may outweigh any benefit gained from more uniform emergence. The tradeoff between planting timing and fieldwork suitable days vs. optimal seedbed conditions will vary depending on climate and geography.

One of the challenges in evaluating the effect of uniform emergence in corn is the fact that field studies have used different methods to measure and express emergence uniformity. On-farm emergence studies conducted by farmers and agronomists commonly measure emergence by time, counting the number of plants that emerge every 24 hours or every 12 hours during the emergence window.

A more accurate assessment of emergence uniformity can be achieved by using temperature data to express emergence timing as a function of growing degree units (GDU). Corn phenology is driven by heat unit accumulation not elapsed time, so difference in GDUs provides a more accurate measure of how far apart two plants are developmentally. Measuring emergence differences by GDUs also helps account for differences in planting timing – for example, a 3-day difference in emergence with mid-April planting would likely represent a smaller difference in GDUs and corn development than a 3-day difference with mid-May planting when temperatures are warmer and GDU accumulation per day is greater.

The best measure of emergence timing is using soil GDUs (sGDU), measured at planting depth, since it most closely measures temperatures that the developing seedlings actually experience in the furrow. However, accurately tracking soil GDUs requires specialized equipment that may not be available or practical in all cases.

Older corn emergence uniformity studies generally did not focus on relatively fine-scale differences in emergence timing within the first few days after the start of emergence, but a pair of recent studies did. The first was a 3-year field study at Ohio State University (partially supported by the Pioneer Crop Management Research Awards program) that assessed effects of soil temperature and moisture flux on emergence timing and uniformity of corn (Lindsey and Thomison, 2020; Nemergut et al., 2021). The second was a 2-year field study conducted by Iowa State University that tested the effects of seed size uniformity and planting depth on corn emergence and yield in conventional and perennial ground cover systems.

The Ohio State study (Nemergut et al., 2021) found that plants emerging within 3 days of the first emerged plants had no per plant yield loss. Plants that emerged more than 3 days after the start of emergence had a 5% decrease in yield per day (Table 1). Adequate planting depth was important for uniform emergence in this study – shallow-planted seeds experienced lower and more variable soil moisture closer to the soil surface, which led to less-uniform emergence.

The Iowa State study (Kimmelshue et al., 2022) produced contrasting results in the two years of the study. The first year of the study (2019) had cold and wet conditions immediately after planting, which delayed the start of emergence, but warmer temperatures once emergence began. Under these conditions, individual plant yield in the conventional cropping system remained stable for the first 5-6 days of the emergence window and then declined linearly after that. In the second year of the study (2020), yield loss was observed with each additional day of delayed emergence after the first day. Yield declined linearly by 7.8 g of yield per plant per day. The researchers attributed this outcome to drought stress that occurred later in the season in 2020. Total growing season precipitation at the Central Iowa study location in 2020 was half of that of 2019. Previous research showed that stress associated with higher plant density increased interplant competition for resources and exacerbated differences in competitive ability among plants (Carter et al., 2001; Maddonni and Otegui, 2004); it’s likely that increased stress due to drought could have a similar impact.

Findings from these two studies show how environmental conditions after emergence can influence the impact of uneven emergence and provide insight into the range of potential outcomes with delayed emergence under different conditions. In the Ohio State study and the first year of the Iowa State study, which experienced relatively normal growing conditions, there was little or no difference in yield for plants emerging within the first few days after the start of emergence. In contrast, the second year of the Iowa State study experienced significant growing season stress, which likely increased competition among plants and magnified the effects of uneven emergence. Under these conditions, individual plant yield began to drop off almost immediately, with plants emerging on the second day of the emergence window already losing yield potential (Table 1).

Table 1. Individual plant yield potential by day of emergence under normal and high stress environments based on results from studies by Ohio State University (Nemergut et al., 2021) and Iowa State University (Kimmelshue et al., 2022).

Corn plants that emerge too late to contribute meaningfully to yield are commonly derided as “weeds,” but is this characterization justified? The designation of a plant as a weed implies that its presence is detrimental to overall yield. A plant that fails to produce an ear while reducing the yield potential of its neighbors in the row by pulling resources away from them would certainly seem to meet the definition of a weed. The key question here is what the impact of the late-emerging plant is on its neighbors.

Plant spacing studies have shown that plants adjacent to a skip in the row can have increased yield due to greater availability of resources. Doerge et al. (2015) reported that plants next to a skip increased their yield by about 10%. Novak and Ransom (2018) found similar results, with an 11% increase in yield for plants next to a skip. For plants adjacent to a late-emerging plant (11-17 days after normal emergence) Novak and Ransom found that there were compensatory increases in yield, but not as much as with a skip – only around 5%. In this scenario, the late-emerging plant would need to produce at least 12% of normal yield to compensate for the yield potential that it is taking away from its neighbors – anything less than this and the late-emerger would be accurately characterized as a weed. Results from emergence timing studies indicate that emergence of a plant would need to be severely delayed relative to its neighbors – likely by 2 weeks or more – before it would cross the threshold of becoming a “weed.”

Recent field studies provide insight into the degree of emergence uniformity necessary to maximize yield potential in modern corn production systems, but how does this compare to emergence uniformity that is currently being achieved?

A Pioneer field study conducted at Johnston, Iowa compared corn emergence timing in continuous corn and a corn-soybean rotation (Figure 5). Emerged plants were flagged and counted each day.

In both cropping sequences, nearly all plants emerged within the first two days and emergence reached 100% on day 3 in the corn-soybean rotation and day 4 in continuous corn (Figure 6).

This study had multiple factors that favored uniformity of emergence – it was planted in a well-managed research field using a research planter travelling at relatively low ground speed. It was also very warm during the emergence window, with 90 GDUs accumulating over 4 days.

In the Iowa State emergence study, the time from the start of emergence (T0) until 95% emergence (T95 ) was 5.28 days in 2019 and 4.25 days in 2020. With some additional time added to account for the last 5% of emergence, the total emergence window in this study was likely around 5-6 days. Nemergut et al. reported time from 10% to 90% emergence (T10-90 ) for the Ohio State study which averaged 3.5-3.8 days (58-62 GDUs). The total emergence window was likely a day or two more than that, which would put it very much in line with the emergence window in the Iowa State study. Across all three studies, emergence windows ranged from approximately 3 to 6 days.

Applying the emergence timing yield outcomes from the two university studies to the emergence data from the Pioneer study provides a look at potential field-level yield outcomes. In two of the three scenarios, there is no yield loss associated with uneven emergence. Under the high stress scenario however, some yield loss would be predicted – 2% yield loss in corn-soybean rotation and 2.7% in continuous corn.

The field studies reviewed here provide insight into corn emergence uniformity currently being achieved with well managed systems under relatively favorable conditions, but what would be the maximum uniformity possible if every part of the planting operation and growing conditions were perfectly optimized? – 2 days? 1 day? Given the widespread interest among corn growers in achieving the shortest emergence window possible, it is important to consider what “success” in this area actually looks like. After all, corn plants are not machines, they are biological organisms that exist in variable, dynamic, and often unpredictable environments. It’s reasonable to conclude that some degree of variability in emergence timing will simply be impossible to eliminate.

Greenhouse or growth chamber studies would seem to offer a potential answer to this question, given their highly controlled and uniform growth environments. An emergence A paper published in 2012 (Egli and Rucker, 2012) included results from multiple greenhouse and growth chamber experiments testing the effects of seed lot vigor on corn emergence uniformity. Emergence uniformity in this study was reported based on the time from 10% to 90% emergence (T10-90 ). The shortest T10-90 times reported from experiments included in this study were 20.4 hours and 24.5 hours. The full emergence windows (T0-100 ) were not reported but, based on the data presented in the paper, can be estimated to be around 40-45 hours. This would be equivalent to around 40 accumulated GDUs based on the reported soil temperatures.

Given that even the most uniform and favorable field environment would be unlikely to match or exceed a greenhouse environment for emergence uniformity, these results suggest that an emergence window of around two days or 40 GDUs is probably the best that could reasonably be achieved in a field environment.

Uniform emergence is important for maximizing corn yield; numerous research studies over the past few decades have clearly demonstrated this fact. However, it is one factor among many with the potential to influence yield, so it must be kept in perspective when prioritizing the allocation of attention and resources in pursuit of greater corn yields. One consistent finding among emergence studies has been that relative emergence timing is often not strongly predictive of individual plant yield (Kovács and Vyn, 2014; Nemergut et al., 2021), demonstrating that emergence uniformity matters, but it is not the only thing that matters. Once the plant is out of the ground, it is subject to numerous other factors, such as moisture, nutrient availability, soil compaction and disease and insect injury that can vary spatially in the field and differentially impact individual plant yield.

Research indicates that an emergence window of 3 or 4 days is sufficient to achieve full yield potential under most conditions and demonstrates that this is an attainable goal in a field environment. Results from greenhouse research suggest that an emergence window of less than 2 days is likely not achievable in a field environment. Much shorter emergence window targets of 12 hours, or 8 hours, or 10 GDUs are commonly touted by corn yield contest growers as essential for maximizing yield potential, but there is no evidence that this degree of uniformity is necessary or even possible; consequently, these targets should not be considered realistic management goals.

• Andrade, F.H., and P.E. Abbate. 2005. Response of maize and soybean to variability in stand uniformity. Agron. J. 97:1263-1269.
• Carter, P.R., E.D. Nafziger, and J.G. Lauer. 2001. Uneven Emergence in Corn. North Central Regional Extension Publication No. 344.
• Doerge, T., M. Jeschke, and P. Carter. Planting Outcome Effects on Corn Yield. Pioneer Crop Insights Vol. 25 No. 1. Corteva Agriscience. Johnston, IA.
• Egli, D.B., and M. Rucker. 2012. Seed vigor and the uniformity of emergence of corn Seedlings. Crop Sci. 52:2774-2782.
• Ford, J.H., and D.R. Hicks. 1992. Corn growth and yield in uneven emerging stands. J. Prod. Agric. 5:185-188.
• Kimmelshue, C.L., A.S. Goggi, and K.J. Moore. 2022. Single-plant grain yield in corn (Zea mays L.) based on emergence date, seed size, sowing depth, and plant to plant distance. Crops. 2:62-86.
• Kovács, P., and T.J. Vyn. 2014. Full-season retrospectives on causes of plantto-plant variability in maize grain yield response to nitrogen and tillage. Agron. J. 106:1746-1757.
• Lindsey, A., and P. Thomison. 2020. Corn Planting Depth: Soil Temperature and Moisture Flux in the Furrow. Pioneer Agronomy Research Update Vol. 10 No. 3. Corteva Agriscience. Johnston, IA.
• Liu, W., M. Tollenaar, G. Stewart, and W. Deen. 2004. Response of corn grain yield to spatial and temporal variability in emergence. Crop Sci. 44:847-854.
• Maddonni, G.A., and M.E. Otegui. 2004. Intra-specific competition in maize: early establishment of hierarchies among plants affects final kernel set. Field Crops Res. 85:1-13.
• Nafziger, E.D., Carter, P.R. and Graham, E.E. 1991. Response of corn to uneven emergence. Crop Sci. 31:811-815.
• Nemergut, K.T., P.R. Thomison, P.R. Carter, and A.J. Lindsey. 2021. Planting depth affects corn emergence, growth and development, and yield. Agron. J. 113:3351-3360.
• Novak, L., and J Ransom. 2018. Factors impacting corn (Zea mays L.) establishment and the role of uniform establishment on yield. Agric. Sci. 9:1317-1336.
• Rotili, H.R., V.O. Sadras, L.G. Abeledo, J.M. Ferreyra, J.R. Micheloud, G. Duarte, P. Girón, M. Ermácora, and G.A. Maddonni. 2021. Impacts of vegetative and reproductive plasticity associated with tillering in maize crops in low-yielding environments: A physiological framework. Field Crops Res. 265:108107.
• Satorre, E.H., Maddonni, G.A., 2018. Spatial crop structure in agricultural systems. Encyclopedia of Sustainability Science and Technology, pp. 1-17.