Although the goal of ensiling forage is to convert actively growing, wilting or crop residue from a wet and unstable condition to a wet and stable state, making it happen and producing quality silage isn’t always easy. Silage-making truly mixes the science (chemistry and microbiology) with art (how efficiently the operation grew the crop and used the proper equipment to harvest, transport and store it) and business (making money from silage and feeding ourselves at the same time), says Steven C. Fransen, Washington State University.
Moisture: Actively growing crops range in moisture content from roughly 40% to 90% given 60% to 10% dry matter (DM), respectively. Because silage ultimately feeds animals that are fed on a DM basis, most silage managers think of silage on a DM basis rather than wet, moisture or as-is basis. Initial moisture at harvest plays a major role in the success of ensiling.
Generally, 65% moisture content (35% DM) is nearly ideal for feedstuffs entering a silo. There is an increased chance of effluent runoff when crop DM is lower than 35%, and with higher DM, the crop may be more difficult to pack and remove air (oxygen) from it at the silo. Either situation is not perfect, but it is very difficult to always hit the target of 35% DM.
Sugars: Another challenge in silage-making lies within the crop. Not all crops are equal in sugars, which are necessary for the lactic acid-producing bacteria (Lactobacillus) to use as energy to produce the acids necessary to lower pH for ensiling. An excellent review paper of sugars in grasses was published by Halford et al. (2010). Grass crops, such as corn, sorghum, wheat, perennial ryegrass and orchardgrass, generally have higher sugar concentrations than legumes, such as alfalfa. Grasses usually have lower protein (N) than legumes. That contributes to more efficient fermentation.
As silage managers, we choose the stage of maturity for the crop to be harvested and ensiled. For corn, the 50% milk line is about 35% DM. Less mature corn at dent stage often ranges between 25% DM and 30% DM. Perennial grasses are harvested multiple times per growing season. Thus, crop DM differs for each cutting. The goal is to cut, wilt and ensile between 25% DM and 35% DM.
At first cutting, when most of the seed heads are produced, we recommend harvesting at boot or very early heading stage. Regrowth cuttings, which are nearly all vegetative growth, are based more on a calendar or field tonnage basis rather than stage of maturity. Alfalfa has higher quality at bud stage, and if allowed to wilt for 24 hours to 48 hours, then the DM will increase to about 30% DM to 35% DM. Alfalfa in the flowering stage is already more mature, so it has higher DM than bud-stage crops.
Bacteria: Lactic acid bacteria (LAB) are necessary for rapid conversion of plant sugars to important volatile fatty acids plus lactic acid. The net result is a drop in pH of the forage mass in the silo. This is necessary for long-term storage and stability of the material.
Tens of thousands of colony-forming units (CFU) of bacteria reside on plant leaf surfaces. The number of CFUs per gram of leaf surface depends on the crop and time of seasonal harvest. Often, there is abundant bacteria to do the job. At other times, bacteria are lacking in abundance or relative activity. It is advisable to supply additional bacteria for effective fermentation.
To read more about the fundamentals of silage, click here.