Sept. 9, 2016 - As corn enters the dent stage, corn silage harvest has started in many areas. Years with below average precipitation and corn that has undergone moisture stress can present some challenges in trying to manage corn silage harvest.
Whole plant moisture
Proper whole plant moisture is critical to good packing and fermentation. Silage that is too wet may not ferment properly and undergo clostridia fermentation, make poor quality feed and may result in excessive seepage. Silage that is too dry may not pack sufficiently for oxygen exclusion for fermentation and result in heating or mould development. Optimum harvest moisture for fermentation will depend on silage storage method (see Table 1, available here).
Moisture may be more difficult to gauge in a moisture stressed crop. Poor kernel development can make it difficult to use milk line as a harvest guide for whole plant moisture, while leaf firing can make plants appear drier than they actually are. Despite this, some growers have actually reported plants being drier than they had anticipated so far this harvest season.
The milk line is the line that separates the hard starch forming at the dent end of the kernel from the soft dough towards the base end of the kernel. The milk line starts with the initiation of denting and progresses towards the kernel base as more starch is packed into the kernels.
While milk line can serve as general guide, it is not necessarily a great predictor of whole plant moisture. Harvest has traditionally been recommended for 1/2 to 2/3 milk line progression, but whole plant moisture may be significantly wetter or drier than optimal depending on the situation (hybrid, weather conditions etc.).
An alternative method may be to use milk line as a proactive indicator of when to start measuring whole plant moisture, such as ahead of traditional harvest recommendations (i.e. 20 per cent progression) (Lauer, J. 1999). This would reduce the risk of starting harvest when whole plant moisture has already become too dry for storage. Actual whole plant moisture can be determined using Koster Tester, the Microwave Test Method, or by submitting a sample as soon as possible to a lab for dry-matter analysis. As a general guide, harvest date could be estimated by assuming whole plant moisture decreases on average 0.5 per cent per day.
Corn silage and nitrates
Excessive nitrate levels in corn silage can be harmful to livestock and increase the risks associated with silo gas.
Nitrate accumulation can occur in plants when there is a large amount of nitrates available in the soil and a stress (i.e. moisture stress) which interferes with normal plant growth and incorporation of nitrates into plant parts such as proteins.
Rainfalls ending an extended dry period can present a risk as renewed moisture can result in a flush uptake of nitrates. If you are considering harvesting after conditions like these, it would be prudent to wait at least five to seven days after a rainfall to allow plants to draw down any spikes in nitrate levels.
The risks are highest with immediate grazing or green chopping corn – nitrate levels may be the highest while concentrations may not be well known. Storage can improve management options as fermentation can reduce nitrate concentrations 25-50 per cent and may present the opportunity to blend with other feeds.
Nitrates concentrations can be test prior to silage harvest, but nitrate concentrations can be highly variable across a field as well as in time, and a sample would need to be representative to what is actually being harvested.
Feed concentrations of nitrate-nitrogen should be less than 1,000 ppm, or 0.44 per cent to be fed without risk. Amounts higher than this should not be fed, or will require careful management, consult with a nutritionist.
Nitrates in silage also contribute towards production of toxic nitrogen oxide based silo gases during fermentation (NO, NO2, N2O4). Risks may be elevated if nitrate concentrations are higher than normal.
Silo gas risks are present every year, but may be elevated if nitrate concentrations are higher than normal. While risk is typically highest with tower silos, caution should be practiced around all silos (gas pockets can form during/after fermentation under silage covers or bags, heavy gas can collect in still, low lying, still areas).
Extra vigilance is always important, particularly given weather the conditions experienced this year. Gases form with fermentation, and may be present shortly after filling (i.e. 12 hours), and are present until they dissipate after fermentation. Be cautious of silos, chutes and silo rooms from the start of filling until two to three weeks after the last load has been filled.