Early evaluation of corn stands is important for identifying yield-limiting factors and making management decisions to mitigate them. Evaluation of corn stands should begin soon after rows are visible, typically 5 to 7 days after the first plants emerge. Corn emergence occurs when 100 to 120 growing degree days have accumulated since planting. As a guide, 8 growing degree days are accumulated on a day with a maximum air temperature of 66°F and a minimum air temperature of 50°F or lower.
When assessing corn stands, evaluate several representative areas in each field while also making note of unusual areas with problems. Documenting emergence problems with GPS and photos tagged with GPS can be useful for future management decisions.
Corn plant population is closely linked to yield and is a key factor when assessing stand establishment. Knowing the plant population relative to the planting rate, soil conditions at planting, and weather conditions from planting to emergence can inform future decisions including planting rate. It is also important for decisions on replanting.
There are many ways to measure corn plant population. With all approaches, accuracy increases when more areas of a field are assessed and a greater number of plants are counted. Only count plants that are healthy and likely to contribute to yield. Counting plants in groups of three can speed the process.
A common approach to measure plant population is to count the number of plants in 1/1000th of an acre and then multiply by 1000 to get plants/acre. The length of row equal to 1/1000th of an acre is 14 feet 6 inches for 36-inch rows, 17 feet 5 inches for 30-inch rows, 23 feet 9 inches for 22-inch rows, 26 feet 2 inches for 20-inch rows, and 34 feet 10 inches for 15-inch rows. When taking stand counts from a length of row, consider counting two adjacent rows to increase sample size.
An alternative way to measure plant population is to use a measuring wheel to measure the length of row with 100 plants and then calculate plant population based on this value. For example, if there were 100 plants in 54.7 feet of row in a field planted to 30-inch (2.5-feet) rows, plant population would be calculated as:
Field trials by the University of Minnesota show that on average, corn grain yield is 96% of the maximum with 26,000 plants/acre, 92% of the maximum with 23,000 plants/acre, 87% of the maximum with 20,000 plants/acre, and 81% of the maximum with 17,000 plants/acre. These yield estimates can vary somewhat with hybrid, soil productivity, expected yield, and other factors. Additional yield losses can occur if there are frequent gaps of missing plants within rows. These can be around 2 to 5% for gaps of 1.5 to 3 feet, and 5% or more for gaps of 5 feet.
Uniformity in emergence
A goal of corn stand establishment is for all plants to emerge at about the same time. This is more important than uniform within-row spacing of plants. Grain yield is reduced by about 20% for plants that are one leaf stage behind earlier-emerged plants early in the season, and by 50% for plants that are two leaf stages behind. Uneven emergence can be caused inadequate soil moisture where the seed was placed, poor seed-to-soil contact, cold soil temperature after planting, soil compaction or crusting, and other factors.
When evaluating corn stands, dig up plants at various locations in a field and assess seedling vigor. Take note of factors reducing seedling vigor, as some of these could be addressed with future management. These can include symptoms of herbicide injury, seedling diseases, insect feeding, salt injury from fertilizers, ammonia toxicity, and nutrient deficiencies.
It is important to assess early-season soil conditions, as these can influence corn growth and uptake of water and nutrients. Notes on early-season soil conditions can be useful for guiding in-season scouting and adjusting future management practices. Look for compacted soil along the seed furrow (sidewall compaction) or below the seed, as this can restrict root development. Sidewall compaction can occur when planter disc openers cut through fine-textured soil that is too wet at planting. Soil compaction just below the depth of preplant tillage can also occur if soils were too wet at the time of tillage.
Soil compaction is of greatest concern for the nodal (or primary) root system of corn. Nodal roots grow from the base of the coleoptile (about 0.75 inches below the soil surface) soon after corn emergence, unlike seminal (temporary) roots, which grow from the seed. At the V3 (three leaf collar) stage, corn plants transition from dependence on seminal roots and kernel reserves to the nodal roots. The nodal roots are the major supplier of water and nutrients from the V6 (six leaf collar) stage to maturity. Since nodal roots develop near the soil surface, they are susceptible to near-surface soil compaction, especially if it is not alleviated by timely rainfall.
It is also useful to note fields with limited residue coverage on the soil surface, as they can be susceptible to wind erosion and sandblasting injury to corn seedlings.
Before replanting, consider whether additional plants are likely to emerge, yield potential of the existing crop, replanting costs, and yield potential of a replanted crop. In Minnesota, corn grain yield is within 8% of the maximum when planting occurs by May 20 to 25.
If replanting is needed, consider whether an entire field should be replanted or only a portion of the field. When replanting thin to non-existent stands, seeding directly into the existing seedbed may be a viable option, especially if it enables an earlier planting date and existing plants are small. In other cases, and when replanting areas with a low but variable plant population, existing plants should typically be removed by appropriate herbicide or tillage. Tillage may also be needed before replanting fields with hardened soils due to heavy rainfall or near-surface soil compaction due to previous field operations.
If replanting, consider the length of the remaining growing season and select hybrids of appropriate maturity. If corn planting is delayed until after the first three weeks of May, switching full-season hybrids to earlier-maturity hybrids reduces the risk of corn freezing in the fall before it has reached maturity. In such instances, planting hybrids that are 5 to 7 or more relative maturity units earlier than those considered full-season reduces the risk of corn being froze in the fall prior to maturity.
Planting grain corn after May 31 carries high risk in Minnesota. However, decisions regarding the crop to plant can be influenced by factors such as fertilizer applied and seed availability. If grain corn is planted after May 31 in Minnesota, growers can reduce risk by planting hybrids that are 15 or more relative maturity units earlier than full-season hybrids.