Watch Corn Silage Moisture,
NOT the Milk Line OR the Calendar

Table of Contents

• When did it silk?
• What about milk line?
• Where will it be stored?
• When to start moisture testing the standing crop?
• How fast will it dry down?
• How many plants to sample? How fine to chop?
• How to check moisture level?
  

"Knee High by the First of July" is a common expression. Have you ever heard the expression "Pollinated by August, in the Silo by Labour Day"?

Areas of the province, which were very dry at pollination, may experience poor grain fill and reduced yields. Much of this corn may go for silage, particularly if hay crop supplies are short. Small ears result in rapid dry down of the whole plant. When normal corn silage is harvested, most of the moisture is in the cob and kernels. When ears are small, the whole plant will be drier than normal.

Proper timing of corn silage harvest is a difficult management decision. When corn is too wet at ensiling, the dry matter and nutrient loss in seepage is high. Seepage is a caustic pollutant. Wet silage will be high in butyric acid (sour tasting), and have poor palatability. Silage that is too dry at harvest will not pack properly. Air will be trapped in the silage, slowing fermentation. Moulds and heat damage are likely. Dry silage will have lower levels of protein, digestibility and Vitamins A and E.

If conditions are dry, watch your corn closely. Dry down will be rapid as well, and may exceed 1% per day in droughty conditions or when grain fill is poor. So, when will it be ready for silage? How can you get it off at the correct moisture for the intended storage and to reduce spoilage? How can you ensure good packing and proper fermentation so that the resulting silage is palatable and high in digestible energy?

When did it silk?

The silking date is the first indicator for predicting date of harvest for silage or grain. Silking to silage harvest is about 42 to 47 days, putting harvest time in the ballpark of the last week in August in the southwest (Table 1). Among hybrids, the grain-filling period (silking to harvest) is quite consistent, but planting to silking will vary considerably.

What about milk line?

When conditions are dry for several weeks prior to harvest, the moisture can be as much as 5 percentage units lower than that predicted by milk line (Table 1). Table 2 shows that moisture in the 1994 crop at two-thirds milk line was wetter than 1995 corn at one-half milk line. In 1995, corn at one-half milk line was already too dry for most silage systems. Variation in moisture at similar milk line also results in significant differences in milk production (Table 2).

Milk line is not a reliable indicator of moisture content. Do not use milk line to determine harvest readiness! This is particularly true for leafy hybrids or during abnormal growing conditions. We have the technology to assess moisture accurately, so let's use it!

Where will it be stored?

You must know the target moisture for the intended storage to ensure a good packing and fermentation in the silo (Table 3). Moisture content is the most important factor to consider to ensure high quality, palatable corn silage.

When to start moisture testing the standing crop?

Although milk line should not be used to assess moisture, it can indicate when to start moisture testing the standing crop. As the kernels start to dent, a separation between kernel starch and milk can be seen. The firm starch is deposited in the crown (outer) layer of the kernel and the milk occupies the basal area towards the tip of the kernel. The milk line (or starch line) can be seen across the kernel shortly after denting. The milk line advances toward the kernel tip with maturity. Once the kernel milk line begins to move down the kernel following the dent stage, the fields intended to be harvested for silage should be sampled for moisture.

The "trigger" on when to start sampling for moisture depends on the intended storage structure (Table 4). Dent is 0% milk line. When the milk line has advanced completely to the kernel tip and a black layer is present, the grain has reached physiological maturity. This takes about 10 to 15 days (Table 1).

For example, a field of corn going into a bunker silo at 70% moisture should be checked for moisture when the "trigger" of 20% milk line is reached in the tip end of broken cobs (Table 4).

How fast will it dry down?

Normal whole plant dry down is about 0.5% per day, but up to 1% per day in hot dry weather. When grain is poorly filled, dry down will be more rapid than 1% per day. For grain, dry down is about 0.75% per day from 30 to 25% moisture and only 0.25% when grain moisture falls below 25%.

If the corn going into a bunker silo tested 74%, then harvest should begin [(74 - 70)/0.5)] = 8 days after moisture testing. In hot dry conditions it should begin [(74 - 70)/1.0)] = 4 days after moisture testing.

How many plants to sample? How fine to chop?

Sample each hybrid and where fields are variable. About 8 to 10 whole plants are needed from each hybrid or field. Corn plants need to be chopped finely (bedding chopper, leaf chipper, forage or silage harvester) for accurate moisture testing. Chopping finely results in less residual moisture, regardless of how the moisture is tested. Even when tested under the best laboratory methods, there will be 1 to 2% residual moisture in the dried sample. This should be considered when determining harvest date.

Recent research and on-farm experiences suggest that we haven't been getting as much "milk" out of corn silage as expected. This is likely because we have been determining when to chop corn based on milk line and calendar date, rather than moisture content. Moisture content is the most important factor in producing high quality, digestible corn silage. Use milk line to determine when to start moisture testing. Use expected dry down rate to determine approximate harvest date. Remember that dry soil conditions will accelerate the dry down rate. Also, every sample has at least 1 to 2% residual moisture after drying. As a final check, sample 1 to 2 days prior to harvest to determine whether the moisture is proper for the intended storage.

Only when an accurate moisture is known can the operator make reasonable management decisions on when to harvest corn for silage.

How to check for moisture level?

Check the moisture levels of your corn silage. Proper moisture at harvesting will allow the corn silage to ensile and store at the optimum level.

The 4 moisture-testing methods are NIR (Near Infrared) moisture, lab oven dry matter, Koster tester and microwave.

Some labs will test moisture using an NIR machine with a corn silage moisture calibration. This is a very accurate method. All labs will offer an oven dry matter which is also quite accurate. In a lab oven analysis, an 80 to 125 gram sample is dried in a 55 C oven overnight. Drying overnight will not remove all the moisture. Even under controlled lab settings, there will be 1.3 to 2.2% residual moisture in the sample after drying in an oven (Figure 1). Sample results from oven drying at a lab will be available in as little as 1 - 2 days.

Koster testers and microwaves can also be used. Researchers' Ballweg and Rankin compared a lab oven, Koster and microwave moisture testing methods in a laboratory setting. The researcher's experienced operator found about 2% residual moisture in the samples (Figure 1). Also, it is important that the sample be finely ground to reduce the residual moisture (Figure 1). This can be achieved by running the whole corn plant into a bedding chopper, leaf chipper or forage harvester. Chopping by hand is not recommended. Fine chopping allows the sample to dry more completely, reducing residual moisture.

Ballweg and Rankin, 1998

Ballweg and Rankin, 1998

An on-farm study looked at residual moisture in samples tested by Koster or microwave using a single experience operator or multiple operators on the same sample of corn silage (Figure 2). The highest residual moisture (6%) was found in microwaved samples using multiple operators. Error levels of this magnitude are unacceptable for growers needing to make a corn silage harvest decision. Ensure that one operator does all the moisture testing, especially when using a microwave (Figure 2). If you are not an experienced operator at using a microwave for moisture testing, use a Koster tester to improve the accuracy of the test.

If you know there will be residual moisture, account for this! For example, if a sample of corn silage tests 37% moisture, there will still be residual moisture in the sample. This moisture could range from 2 to 6%, depending on the testing method and the experience of the operator. This means the actual moisture of the sample is 39 to 43%!

Koster testers are available from many farm supply dealers in Ontario. Using a 100-gram sample in a Koster tester will take over 30 minutes if the silage is very wet. When you think the sample is dry, weigh it. Return it to the Koster for 2 to 3 additional minutes, then weigh it again. If the moisture has not changed, the sample is dry (assume at least 2% residual moisture). Digital postal scales are more accurate than the scale provided with the Koster tester.
For microwave testing, weigh and record the weight of an empty paper plate or bag. Record this weight as A. Spread an 80 to 125 gram sample evenly on the plate or bag. Weigh and record the exact weight (sample plus container). Record this weight as B. Don't forget to put a glass of water in the microwave. Dry on high power for 2 minutes (or up to 3 to 4 minutes if the sample is very wet). Remove, record weight, gently stir the sample and return it to the microwave. Don't burn your fingers! Continue for intervals of 30 seconds to 1 minute on medium-low until the sample weight is stabilized. When the weight remains the same for 3 drying intervals, record the weight as C. If the sample becomes charred, start again with a fresh sample.

To calculate moisture, use the following formula:

[(B - A) - (C - A)] ÷ (B - A) x 100 = % Moisture
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Accuracy of Moisture Testing
Fine > coarse (Figure 1)
Lab oven > Koster > microwave (Figures 1 and 2)
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Accurately determining the moisture of corn silage on the farm is a difficult process. A consistent protocol is essential. Even with a good protocol, residual moisture will be at least 2%. A better option is to submit samples to a lab for an oven dry matter analysis. A lab analysis reduces sample-to-sample variability and improves accuracy. When an accurate moisture is known, the operator can make reasonable management decisions on when to harvest corn for silage.