The first step in handling drought stressed corn is to determine the success of pollination. Two techniques are commonly used to assess pollination success or failure. The most rapid technique to determine pollination success is the "shake test." Carefully unwrap the ear husk leaves and then gently shake the ear. The silks from fertilized ovules will drop off. The proportion (%) of silks dropping off the ear indicates the proportion of future kernels on an ear. Randomly sample several ears in a field to estimate the success of pollination.

The second technique is to wait until 10 days after fertilization of the ovules. The developing ovules (kernels) will appear as watery blisters (the "blister" R2 stage of kernel development).

If pollination is good, harvest in a normal fashion for either grain or forage use. If pollination is poor yet some kernels are developing, the plant can gain dry matter and farmers should wait with harvest. In Wisconsin, many farmers have the option of harvesting poorly pollinated fields for silage use. If there is no pollination, then the best quality forage will be as found as close to flowering as possible. Quality decreases after flowering. The challenge is to make sure that no potential pollination occurs and that the forage moisture is correct for the storage structure.

Forage quality of normally pollinated corn

Corn has two peaks in forage quality: one at pollination and one at harvest maturity. The early peak in forage quality at pollination is high in quality but too wet for ensiling. The later peak is more familiar, and is the one we typically manage for when producing corn silage.

Forage Quality of Poorly Pollinated Corn

Coors et al. (1997) evaluated the forage quality of corn with 0, 50 and 100% pollination of the kernels on an ear during 1992 and 1993. These years were not considered "drought" stress years, but they can give us an idea as to quality changes occurring due to poor pollination. These plots were harvested in September.

A typical response of corn to stress is to reduce grain yield. Bareness reduced whole-plant yield by 19% (Table 1). Kernels on ears of 50% ear fill treatments were larger and tended to more than make up for reduced numbers (Albrecht, personal communication). With the exception of protein, as ear fill increased, whole-plant forage quality increased.

Table 1. Forage yield and quality of corn with differing amounts of pollination grown at Madison in 1992 and 1993 (n= 24).
Ear fill	Forage yield	Crude protein	NDF	ADF	IVTD	NDFD
%	% of control	%	%	%	%	%
0	81	8.5	57	30	74	52
54	93	8.0	54	28	76	52
100 (control)	100	7.5	49	26	77	54
LSD (0.05)	6	0.3	1	1	1	1

A few management guidelines for handling cornfields with poor pollination:

Forage moisture

If the decision is made to harvest the crop for ensiling, the main consideration will be proper moisture for storage and fermentation. The crop will look drier than it really is, so moisture testing will be critical. Be sure to test whole-plant moisture of chopped corn to assure yourself that acceptable fermentation will occur. Use a forced air dryer (i.e. Koster), oven, microwave, electronic forage tester, NIR, or the rapid "Grab-Test" method for your determination. With the "Grab-Test" method (as described by Hicks, Minnesota), a handful of finely cut plant material is squeezed as tightly as possible for 90 seconds. Release the grip and note the condition of the ball of plant material in the hand.

If juice runs freely or shows between the fingers, the crop contains 75 to 85% moisture.
If the ball holds its shape and the hand is moist, the material contains 70 to 75% moisture.
If the ball expands slowly and no dampness appears on the hand, the material contains 60 to 70% moisture.
If the ball springs out in the opening hand, the crop contains less than 60% moisture.

The proper harvest moisture content depends upon the storage structure, but is the same for drought stressed and normal corn. Harvesting should be done at the moisture content that ensures good preservation and storage: 65-70% in horizontal silos (trenches, bunkers, bags), 60-65% in upright stave silos, and 55-65% in upright oxygen limiting silos.

Raising the bar

Depending upon farm forage needs, raising the cutter-bar on the silage chopper reduces yield but increases quality. For example, raising cutting height reduced yield by 15%, but improved quality so that Milk per acre of corn silage was only reduced 3-4% ( Wisconsin). In addition the plant parts with highest nitrate concentrations remain in the field (Table 2).

Table 2. Nitrate concentrationof corn plant parts.
Plant part	NO₃N
	ppm
Leaves	64
Ears	17
Upper 1/3 of stalk	153
Middle 1/3 of stalk	803
Lower 1/3 of stalk	5524
Whole plant	978
Derived from Hicks, Minnesota

Nitrate problems

If drought-stressed corn is ensiled at the proper moisture content and other steps are followed to provide good quality silage, nitrate testing should not be necessary. The risk of nitrate poisoning increases as pollination becomes poorer. Nitrate problems are often related to concentration (i.e. the greater the yield the less chance of high nitrate concentration in the forage). If pollination is poor only about half of the dry matter will be produced compared to normal corn forage.

It is prudent to follow precautions regarding dangers of nitrate toxicity to livestock (especially with grazing and green-chopping) and silo-gasses to humans when dealing with drought-stressed corn. Nitrates absorbed from the soil by plant roots are normally incorporated into plant tissue as amino acids, proteins and other nitrogenous compounds. Thus, the concentration of nitrate in the plant is usually low. The primary site for converting nitrates to these products is in growing green leaves. Under unfavorable growing conditions, especially drought, this conversion process is slowed, causing nitrate to accumulate in the stalks, stems and other conductive tissue. The highest concentration of nitrates is in the lower part of the stalk or stem. If moisture conditions improve, the conversion process accelerates and within a few days nitrate levels in the plant returns to normal. Nitrate concentration usually decreases during silage fermentation by one-third to one-half, therefore sampling one or two weeks after filling will be more accurate than sampling during filling. If the plants contain nitrates, a brown cloud may develop around your silo. This cloud contains highly toxic gases and people and livestock should stay out of the area. The resulting energy value of drought-stressed corn silage is usually lower than good silage but not as low as it appears based on grain content. The only way to know the actual composition of drought-stressed corn silage is to have it tested by a good analysis lab.

Marshfield Plant and Soil Analysis Laboratory
8396 Yellowstone Dr.
Marshfield>, WI 54449-8401
Phone: (715) 387-2523

Estimating yield

Growers need to carefully monitor, inspect and dissect plants in their own fields as to plant survival potential, kernel stages, and plant moisture contents in determining when to begin silage harvest. Fields and corn hybrids within fields vary greatly in stress condition and maturity. Often questions arise as to the value of drought-stressed corn. In order to estimate pre-harvest silage yields, the National Corn Handbook publication "Utilizing Drought-Damaged Corn" describes methods based on either corn grain yields or plant height (if little or no grain yield is expected). Below is a summary of this publication.

Grain yield method for estimating silage yield: For moisture-stressed corn, about 1 ton of silage per acre can be obtained for each 5 bushels of grain per acre. For example, if you expect a grain yield of 50 bushels per acre, you will get about 10 tons/acre of 30% dry matter silage (3 tons/acre dry matter yield). For corn yielding more than 100 bushels per acre, about 1 ton of silage per acre can be expected for each 6 to 7 bushels of grain per acre. For example, corn yielding 125 bushels of grain per acre, corn silage yields will be 18 to 20 tons per acre at 30% dry matter (5 to 6 tons per acre dry matter yield). See also Table 2 in A1178 "Corn silage for the dairy ration."

Plant height method for estimating silage yield: If little or no grain is expected, a rough estimate of yield can be made assuming that 1 ton of 30% dry matter silage can be obtained for each foot of plant height (excluding the tassel). For example, corn at 3 to 4 feet will produce about 3 to 4 tons per acre of silage at 30% dry matter (about 1 ton per acre of dry matter).

References

Coors, J. G., Albrecht, K. A., and Bures, E. J. 1997. Ear-fill effects on yield and quality of silage corn. Crop Science 37:243-247.

Utilizing Drought-Damaged Corn (NCH-58) www.agcom.purdue.edu/AgCom/Pubs/NCH/NCH-58.html

Weather Stress in the Corn Crop (NCH-18) www.agcom.purdue.edu/AgCom/Pubs/NCH/NCH-18.html

Growing Season Characteristics and Requirements in the Corn Belt (NCH-40) www.agcom.purdue.edu/AgCom/Pubs/NCH/NCH-40.html