Corn Silage Harvesting

With this summer's history of wet weather and unpredictable temperatures, it is anyone's guess as to what awaits the dairy producer during the corn silage harvest. A prolonged harvest may result in a mature and extremely dry crop. An early frost may result in an immature crop. Each of these scenarios will provide harvest and subsequent feeding challenges to Manitoba's dairy producers.

To understand these challenges better, it is necessary to have an understanding of "normal" changes during the maturation process. As the corn plant matures, there is an increase in grain content. At the milk stage, the plant is comprised of approximately 10% grain as compared to 30% at 1/3 milkline and almost 50% grain at physiological maturity. The choice of corn hybrid can influence these percentages. Along with the increasing grain (and starch) content, there is a corresponding decrease in whole plant ADF and NDF due to the dilution effect. Despite increasing starch and decreasing fibre components, mature corn silage has lower digestibility of both the grain and fibre components. Starch becomes less available as kernels dry down, harden and become more resistant to digestion. Forage becomes less digestible as the sugars in the stalks and leaves move into the kernel during grain filling and are converted to starch. The extent of the changes in digestibility depends on the hybrid grown. Some hybrids maintain the digestibility of their stover fraction as the plant matures, while the digestibility in other hybrids decreases rapidly as the corn plant approaches physiological maturity. Selection of a corn hybrid that maintains stover quality is a logical management choice.

When is the ideal time to harvest corn silage? Past recommendations have been based strictly on milkline with the recommendation to harvest at 1/2 to 2/3 milkline. Recent work has shown that milkline is not always a good predictor of whole plant moisture and that substantial variation can occur. For example, in Wisconsin in 1994, 2/3 milkline corn silage was 64.9% moisture. Following the hot, dry summer of 1995, one-half milkline silage had a moisture content of 60.3%. University of Wisconsin dairy researchers recommend beginning harvest at 70% moisture and completing before 65% moisture if the silage is going into bunker silos or bags. This results in a good compromise between grain content, sugar content, stover digestibility and moisture content. To prevent excessive seepage, silage going into top unloading uprights should have an average moisture of 62-65% and not less than 60% moisture if the corn is going into sealed uprights. Whole plant moisture can be accurately determined on-farm using a microwave or Koster moisture tester.

A delayed or extended harvest season will result in a crop which has passed the "ideal" moisture and maturity stage. Digestibility of both the fibre and grain will be lower thus lowering overall feeding value. Research has shown that corn silage harvested at the black layer stage produces significantly less milk than corn silage harvested at ½ to 2/3 milkline. Another problem with a mature crop is the low moisture content. Adequate moisture (more than 40%) is essential to ensure good packing and exclusion of oxygen from the silo. Lower moisture contents will result in poor fermentation, heat damage, poor bunklife and poor intake. Even corn silage with only 55% moisture can be difficult to ensile. Several options may be considered to "salvage" some feed value from a dry crop. Storing the crop in AGBAGs is an effective way to remove oxygen and improve fermentation. Anhydrous ammonia injected into a covered pile of plant material will kill aerobic bacteria and improve fermentation and stability.

Research has shown that the fermentation and digestibility of mature corn silage can be improved through processing prior to ensiling. Harrison et al, from Washington State University observed an increase of one kilogram of milk per cow per day from black line corn silage harvested with an on-board mounted kernel processor compared to black line corn silage harvested with a conventional chopper. This is slightly less than the increase observed when feeding ½ milkline silage versus black line silage harvested with conventional choppers. In general, mechanical processing of mature whole plant corn can result in a forage that ensiles faster, has less dry matter loss during ensiling and has increased digestibility of starch and fiber.

One of the biggest challenges associated with mature corn silage is assigning a net energy of lactation (NEL) value which truly reflects the energy content. NEL values are typically calculated from ADF analyses. The lower the fibre, the higher the NEL value. The equations do not factor in the decrease in both fibre and grain digestibility. Because of this, current prediction equations tend to overvalue corn silage harvested at the black line maturity. Unfortunately the analytical capabilities of most commercial labs means that the tests needed for more accurate energy determination (lignin, starch, digestibilities) are not routinely available. Equations have been published which adjust the NEL of corn silage based on DM and therefore maturity. For example, a silage harvested at 2/3 milkline may have a DM of 35% and an ADF of 26 %. A silage harvested at black layer may have a DM of 45% and an ADF of 25%. The use of standard prediction equations would result in both silages having the same NEL. Use of the adjusted equations would result in NEL values of 1.6 and 1.3 mcal/kg for 2/3 milkline and blackline forages, respectively.

Mature, dry silages are prone to heating and subsequent protein damage. Once heating has occurred there is little that can be done, other than recognizing the problem and formulating the ration accordingly. It is recommended that dry silages (less than 50% moisture) be tested for heat damage. The protein value of the silage can then be adjusted before balancing the ration.

Another possible scenario for this year's corn silage harvest is a crop which is subjected to frost at or before the ideal maturity. The yield effect of frost depends upon the degree of damage and the stage of development. Expect grain yield losses of approximately 5% if a corn plant at ½ milkline experiences only a light frost. A plant at the same stage of development, which experiences a killing frost, will have losses of 10-15%. Low starch fill becomes a problem when immature plants are killed by frost. The decrease in NEL is often not as large as might be expected though because of the higher digestibility of both the grain and stover. However, growing conditions can further impact fibre digestibility. Wet and cold conditions can cause up to a 20% reduction in fibre digestibility while reported ADF and NDF levels remain "normal". Given these factors, assigning an accurate NEL value to immature corn silage is as difficult as assigning an accurate NEL to mature silage. Some nutritionists limit the value of immature corn silage to 1.6 mcal/kg even though higher values may be reported on the feed analysis report.

Another feeding challenge with immature silage is dealing with the different partition of nutrients (less starch, more sugars). Particular attention will need to be paid to NSC levels when formulating rations to ensure rumen digestion is not affected.

Any crop, regardless of maturity, which is rained on or left in the field long after it should have been harvested, will be prone to high levels of mold and yeast. This will have negative effects on the ensiling process, bunk life and palatability. The use of anhydrous ammonia may be an option to salvage some feeding value.

In summary, dealing with mature forages, or those left to stand for a prolonged time after a killing frost, should be viewed primarily as salvage operations. Difficulties in ensiling can be expected. Determining the true NEL content of these silages is a challenge but one that must be met to minimize impacts on milk production. There is one harvest scenario not yet discussed. The sun will shine and Manitoba producers will harvest a bumper crop of high quality corn silage! Good luck to all!