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Water is extremely important for crop production. When water becomes limiting to the plant it is important to understand how plants use water. We often hear the term evapotranspiration (ET) in relation to plant water demand. ET is a combination of soil water evaporation (E) and water used by the plant during transpiration (T). Soil evaporation is the major loss of water surface and typically is higher after rain and under high temperature conditions.
Dry conditions in some areas of Iowa this summer are resulting in quite variable corn growth and production potential. Some livestock producers are considering harvest of corn damaged by drought conditions for silage. Corn silage harvest results in more phosphorus (P) and potassium (K) removal than grain alone because almost the entire plant is harvested. The increased amount removed with silage differs for P and K because the relative amount of P and K is different in corn vegetative parts than in grain.
The 2017 growing season for much of Iowa so far has been characterized by low rainfall and high temperatures. This has generated concerns about water stress and yield reductions. In particular, 2017 maximum temperatures are 2-6 percent above average and precipitation 4-75 percent below average (equivalent to 0.3 to 5 inches deficit) from June 1 to July 15 (Figure 1). Radiation is 5-15 percent above average across Iowa. Minimum temperatures and growing degree days (GDD) are below average in northern parts of Iowa and above average in the central and southern parts of Iowa.
The problems experienced with off-target movement and injury in states south of Iowa, particularly Missouri, Arkansas and Tennessee have been well covered in the press, but there has been little discussion of problems in Iowa. Unfortunately, the lack of press does not mean everything has gone smoothly in the state. While I don’t know the percentage of Xtend soybean fields experiencing problems with off-target movement, in my mind the number is too high.
Bacterial leaf streak was confirmed for the first time in Iowa and other states in 2016. There is not a lot known about the disease but researchers at Colorado State University, Iowa State University, Kansas State University and University of Nebraska are collaborating to understand the disease and its impact on corn with partial funding through the Farm Bill.
Crop water use (transpiration) during the growing season is a major factor in attaining high yield potential. Soil water loss (evaporation) and crop water loss (transpiration) occur simultaneously; making predictions of evapotranspiration complex. Actual evapotranspiration values vary greatly from day to day (0.04 to 0.40 inches/day) because of the following factors:
Corn roots grow rapidly starting at the 4th-leaf stage and continue throughout vegetative development. This typically occurs from June to early July. Several factors affect root growth, but temperature and soil moisture are the most relevant factors in the absence of soil constraints. Well-developed, deep root systems are essential to support water and nutrient uptake and thus high yield potential. Hot and dry weather results in a depletion of moisture in the top 6-inch soil layer.
Oak tatters is a disorder that primarily affects white oaks, but also is observed on hackberry trees. Leaves of affected trees lose the majority of interveinal leaf tissue, resulting in a leaf ‘skeleton' (Figure 1). The phenomenon was first reported in the early 1980’s, and has been observed in many Midwestern states. The number of trees affected varies widely from year to year, with a much higher level of incidence in 2017 than normal. The disorder occurs in both rural and urban areas and may affect single trees and those in woodlands. Symptoms usually are distributed uniformly throughout
Japanese beetle is an invasive insect capable of feeding on corn and soybean. This pest has been in Iowa since 1994 but its distribution in field crops is sporadic around the state. Statewide populations have been low since 2014 and it is unclear if pressure will be significant this year. Japanese beetle adults need about 1,030 growing degree days (base 50°F) to complete development and will continue emergence until around 2,150 degree days. Based on accumulating degree day temperatures in 2017, Japanese beetle adults should be active in some areas of southern Iowa this week (Figure 1).
Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average peak hatching date of June 6 in central Iowa. In 2017, the average hatching date will be about the same time as the 2014-2016 growing seasons. Development is driven by soil temperature and measured by growing degree days. Research suggests about 50 percent of egg hatch occurs between 684-767 accumulated degree days (base 52°F, soil). Most areas in Iowa have reached peak corn rootworm egg hatch or will within a few days (Figure 1).
