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Determining the proper time to harvest corn for silage is important to obtain a high quality forage. You can begin to predict when a field will be ready for silage harvest by knowing the tasseling/silking date.
Corn silage needs to be harvested at the proper moisture content for storage and preservation of a quality product. Good silage fermentation and animal performance generally occurs when corn is harvested and preserved between 30 and 40% dry matter (DM) or 60 to 70% whole-plant moisture. The optimal DM content will vary depending on the type of storage structure. Ensiling corn silage that is too wet produces poor fermentation, seepage losses, and lowered animal feed intake. If it is too dry, there is the risk of mold development, heat damage, and lower whole-plant digestibility.
Figure 1. Corn silking date can be used as a rough indicator of which fields are likely to be the earliest to harvest for silage.
Predicting when to harvest corn for silage to achieve proper %DM for ensiling can be difficult. There are no easily identifiable plant traits that can be used to estimate %DM. Observing the development of the corn kernel milkline has been used to estimate when corn is at the proper DM content for ensiling, with recommendations of 1/2 to 3/4 of the way down the kernel. However, this has not been found to be a very accurate or reliable guide to gauge whole-plant DM content. Likewise, timing corn silage harvest based on a heat unit system can also be unreliable.
The first step in predicting when to harvest corn for silage is to note the silking date of the crop in a field. Silking occurs a couple of days after tasseling. A general rule of thumb when taking flowering notes is that once the field is 80-90% tasseled, you can assume you are about 45 days to harvest for silage. Other indicators are that the dent stage occurs about 35 to 42 days after silking, and kernel half milkline typically occurs about 42 to 47 days after silking.1,2 Silking date can be used as a rough indicator to begin staging fields for silage harvest.
Fields should begin to be checked about 2 weeks after tasseling to gauge the maturity of the crop. Once the dent stage (R5) occurs, the field should be checked for kernel milk stage development. Once the kernel milkline begins to move, you should start sampling for whole-plant moistures.
Sampling the crop and measuring the %DM of whole plants is the only reliable method of determining the optimal time to harvest corn for silage. When corn reaches the dent stage, start to take whole-plant samples and process them to accurately assess moisture levels. Whole-plant moisture can be run by a lab or you can do it on the farm with a microwave. Once you have a whole-plant moisture reading, you can use a constant factor of 0.5% drydown per day to predict how many days to harvest. For example, if the whole-plant moisture was 80% and your target moisture is 70%, estimated days to harvest would be 20 days (80% - 70% divided by 0.5).
The procedure will provide a rough estimate for the harvest date. There are many factors that can affect the drydown rate which can range from 0.5% to 1% per day. Therefore, you should sample the field again as the predicted harvest date approaches to determine whole-plant %DM prior to chopping.