By Allan Brax

Pioneer Hi-Bred agronomist

Second-generation European corn borer moth flight has started in northeast Colorado. Egg laying is expected in the near future. Corn borer egg masses are generally found on the underside of the leaf usually within three leaves above or below the ear leaf. These egg masses appear flat and white with black spots appearing as the eggs mature.

Generally, late-planted and late-maturing cornfields are more attractive to egg-laying moths than early-maturing fields. Second-generation flight can last for four weeks. Damage due to second brood in corn may contribute to lodging and eardrop losses.

The simple method used to determine economic threshold is based on accumulation of the percentage of plants with egg masses during two consecutive weeks of scouting following the early portion of the egg-laying period. If the first and second scouting show 25 percent of cumulative plants with egg masses, treatments should be made. This percentage should be raised to at least 50 percent after milk stage.

Pollination period and watering tips. Moisture stress during pollen shed and silking can cause more yield loss than any other developmental stage in corn. The following tips are provided to avoid unnecessary moisture stress on your irrigated crop:

Silk through milk = maximum daily water use (2.3 inches per week). Since this cannot be supplied through irrigation alone, ensuring soil moisture profiles are full prior to this peak demand period is critical.

Evapotranspiration (moisture loss through heat + transpiration, or water loss through the plant's leaves) peaks during this same period because the reproductive stage of the crop (highest water-use period) coincides with the hottest temperatures of the season. Avoid irrigation "off-time" unless significant rainfall volumes are observed.

Corn's natural response to heat is to "roll up" as its only defense against burning up. Fields under pivot irrigation must supply adequate volumes of water - even if rolling is observed. Quick passes to relieve visual plant stress from heat will be more harmful overall simply because moisture applied at inadequate volumes will have a higher percentage of evaporation, forcing depletion of soil-stored moisture much more rapidly.

Plant height and yield concerns. Remember, plant size is not an indicator of yield. Just because a plant is shorter in stature does not mean its yield potential is lowered. Plant height, more specifically in-season plant height, is influenced by a couple of primary factors.

Genetics.

Temperature. In corn, rate of growth is a function of temperature, where adequate moisture is present. Maximum growth occurs at 80 to 85 F, rapid growth at 74 F and slow growth at 65 F.

The causes of shorter-than-normal corn can be traced back to planting date and temperature during stalk elongation. Cooler temperatures in the early vegetative stage (as in 2004) equates to shorter plant height - i.e., the two- to three-week period in mid-May when temperatures were significantly lower than normal. Much of the early-planted corn was beginning or well within the stalk elongation period. This extended period of cool temperatures influenced the elongation of internodes in the lower third of the stalk and accentuated the expected shorter heights of early-planted corn.

Editor's note: Crop Talk is brought to you by agronomists serving your community to keep you informed about current growing conditions and important management issues. Allan Brax, Sterling, Colo., is an agronomist with Pioneer Hi-Bred International, Inc.

Date: 8/24/05