One of the most frequent questions I hear regarding drought-stressed forages is in reference to harvest time. When should I harvest? This is good because it’s probably the most important question you should be asking.
This is what will determine whether or not you will be making lemonade with your lemons. There are definitely rules of thumb we’ve all heard, such as waiting for about 67 percent of the leaves to turn brown or harvesting when the milkline is 70 percent of the way down to the kernel tip.
Although these adages may get you by in a pinch, they are not the best methods for determining when to harvest your corn silage because much of the moisture is located in the stalk, and there are fewer living leaves for evaporative transpiration or moisture loss to occur (i.e., the plants will generally be wetter than you think).
In addition, drought-stressed corn generally has reduced kernel production if the plant has even produced ears at all, making it more difficult to determine harvest time by inspecting the kernels.
When should I really harvest?
The best technique for determining the proper time to harvest is to collect three to four plants from each field and determine the whole-plant moisture. When moisture concentration is between 60 and 70 percent, start harvesting. Of course, whole-plant moisture is critical whenever we put up wet forages for ensiling, and that is no different in a drought-stress situation.
There have been situations when mowing and wilting of plants, with few or no leaves that are too short to chop, has been suggested to help dry them to the desired moisture concentration. This practice will achieve its goal but increases the risk of introducing undesirable bacteria from the soil, such as clostridia, that may have a negative effect on fermentation of the silage.
Using a silage inoculant is always advisable when putting up wet forage, especially when ensiling a drought-stressed forage because the risk of putting up forages at a sub-optimal moisture concentration is greater than normal. Additionally, drought-stressed silage may have a greater sugar concentration than typical corn silage after the ensiling process is complete. This means there is more “food” available for aerobic micro-organisms to cause spoilage during feedout. This is not a time to get stingy or cheap with inoculants.
What about feed quality?
The second question that comes up a lot is around the feeding quality of the resulting silage. Granted, there won’t be as much starch as normal corn silage (if any), so the silage will be lower in energy.
However, don’t forget that in drought situations, the plant will partition more energy into highly digestible fiber and sugars within the stalk and leaves which, as some research suggests, contributes to a feeding value about 70 percent of non-drought-stressed silage. It is always important to test your silage for nutrient concentrations but is highly advisable with drought-stressed corn silage because of the increased variability that comes along with these forages.
What about nitrates?
There’s usually a lot of discussion centered on greater nitrate concentrations in drought-stressed forages, especially in grasses. This is a legitimate concern as drought conditions will concentrate nitrates in the lower portion of the stalk, enough to cause possible toxicity. Corn grown in fields fertilized with nitrogen or manure are more susceptible to nitrate accumulation.
However, it is possible to successfully manage this risk. The first step would be to increase the cutter height above 10 inches. This will decrease your yield, but it also reduces the amount of nitrate you harvest. If it rains, avoid harvesting drought-stressed forages within three to four days after the rainfall.
The moisture will cause the plant to take up more nitrogen from the soil. It also will take about three to four days for the plant to convert the nitrogen into proteins, thus reducing nitrate concentration.
The second step is the ensiling process itself. On average, ensiling can reduce nitrate concentration by as much as 50 percent, and using a good inoculant can help to significantly decrease nitrate concentration even further.
With this second point in mind, it is a good idea to avoid feeding drought-stressed greenchop because of the greater nitrate concentration in this feed than silage. Greenchop that has been sitting will have greater microbial activity, thus converting nitrates to the more toxic form, nitrites.
Drought isn’t the only stressor that will cause increased nitrate accumulation in the plant. If the crop has been stressed via hail, cold, disease or other events, obtain a nitrate analysis so you and your nutritionist know what you are feeding. High-nitrate forages can still be successfully incorporated into your ration as long as they are fed with other ingredients containing no, or very little, nitrates.
Don’t forget to account for the concentration of nitrates in your water supply as well. Remember, rumen microbes are able to detoxify nitrates, but they need time to adapt to greater nitrate concentrations.
Stressed forage should be introduced and increased in the diet slowly, keeping in mind there is a finite amount of nitrate that can be fed before problems begin to present themselves. The tolerable concentration of nitrate in the diet is different depending on which animals in your herd you feed the forage.
It is a good practice to avoid feeding high-nitrate forages to more susceptible animals, such as pregnant and transition cows.
Harvesting and feeding drought-stressed forages can seem like a daunting task. However, with the right attitude and proper planning, you can successfully manage and utilize what may have otherwise seemed like a failed crop and maximize its value. 
PHOTO: Sometimes, despite all of our best efforts, we end up dealing with drought-stressed forages. This article will help guide you from harvest to feeding. Staff photo.
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Benjamin Pamp
- Dairy Technical Services
- Cargill
- Email Benjamin Pamp
