Wednesday, August 19, 2009
Harvest Strategies to Optimize Corn Silage Quality and Yield
With the majority of the Minnesota corn crop in the milk stage (Figures 1 and 2), now is a great time to begin planning for corn silage harvest. Proper harvest management is critical for high quality silage, and it starts with harvest timing. This ensures that the harvested crop is at the optimum moisture for packing and fermentation. Silage that is too wet may not ferment properly and can lose nutrients through seepage. If silage is too dry when harvested, it has lower digestibility because of harder kernels and more lignified stover. In addition, dry silage does not pack as well, thus increasing the potential for air pockets and mold.
Optimum silage moisture at harvest ranges from 50-60% for upright oxygen limiting silos, 60-65% for upright stave silos, 60-70% for bags, and 65-70% for bunkers (Lauer, 2003). Due to variability among hybrids and growing conditions, it is necessary to measure silage moisture using a commercial forage moisture tester or microwave oven rather than simply estimating it from the kernel milkline. Instead, kernel milkline should be an indicator of when to collect the first silage samples for moisture testing. A general guideline is to begin moisture testing when the milkline is 25% of the way down the kernel for horizontal silos, and 40% of the way down the kernel for vertical silos. Then, assume a constant drydown rate of approximately 0.6% per day, and measure moisture again prior to harvest (Roth, 2003).
Length of cut and crop processing are also important for obtaining high-quality corn silage. This is because breakage of cobs and kernels increases surface area, which improves digestibility, reduces cob sorting, and results in higher density silage that packs better. Although crop processors are expensive, the higher-quality silage that they produce can increase milk production by 300 pounds per cow per year (Schroeder, 2004). The benefit to crop processors is greatest when there are harder kernels resulting from delayed harvest or drought. When using a crop processor, chopper cut length can be increased, thereby reducing horsepower requirements while maintaining optimum particle size. For example, ideal chop length is 0.375 inch theoretical length of cut for unprocessed corn (Schuler, 2000). For processed corn, recommended settings are and 0.75 inch theoretical length of cut with 0.08 to 0.12 inch roll clearance.
A 4 to 6 inch cutting height is generally recommended for corn silage, as it maximizes silage yield and milk per acre. However, drought-stressed corn can accumulate nitrate in the lower part of the stalk, thus increasing the potential for nitrate poisoning, particularly in older livestock on lower-energy rations. The potential for high-nitrate silage can be even worse if drought-stressed silage is harvested within 10 days of rainfall, since rainfall increases crop uptake of soil nitrogen.
Silage with high nitrate levels can be managed by dilution with other sources of feed or by increasing the cutting height to 12 inches. Silage cut at this higher height has been shown to have 8% less silage yield and 2% less milk per acre (Lauer, 2003). This same study found that a cutting height of 18 inches resulted in 15% lower silage yield, 12% higher milk per ton, and 4% lower milk per acre when compared to a 6 inch cutting height. Increased silage quality with high cutting is due to a higher ratio of grain to stover. However, corn stalks are a good source of fiber and the lower tonnage with high-chop silage typically makes it hard to justify in the absence of high nitrate levels.
When harvest begins, fill silos rapidly to reduce exposure of silage to oxygen and to reduce fungal growth. For bunker silos, pack silage as tightly as possible in progressive wedges in depths of 6 inches or less.