There’s a lot going on in the bunker that’s filled and ready for feed-out throughout the coming months, and much of what’s happening inside has already been predetermined by the crop and harvest for that year. The dairy farmer’s annual goal is to grow, harvest and store an ensiled product that will ultimately feed cows so they’ll perform well, but there are numerous roadblocks to the perfect silage.
Dr. Robert Van Saun, Penn State veterinarian and nutritionist, says nutrition is the single most important thing on the farm from both a health and a productivity standpoint. He explained there are six goals for stable silage: rapid pH decline to an optimum level, achieving the proper spectrum of fermentation acids, conservation of water soluble carbohydrates, minimizing protein degradation, controlling fermentation temperature and minimizing aerobic activity upon feedout.
“When you harvest forage in the field, that forage is not sterile,” said Van Saun. “It has bacteria, weeds and molds growing on the plant. If we cultured a plant out in the field under optimum conditions, we could find, on average, 300,000 colony-forming units of mold and bacteria per gram.”
The problems with high yeast counts or molds and mycotoxins begin when that number is close to one million colony-forming units per gram. “Molds and yeast are unique in that they need oxygen,” said Van Saun. “So that’s one of the other primary things, and the biggest nemesis to making good quality forage is that you have to get the oxygen out of the material as it’s packed into the silo. That’s why we talk a lot about packing.”
But proper packing is more than a function of the tractor weight. Van Saun says particle length and dry matter (DM) content are the key factors, and that’s why harvesting at the proper maturity and moisture level is important. “We may need to modify the process if that plant is too mature and something went awry that year,” he said, “like in a drought year.”
Low pH prevents the bacteria, yeasts and molds from growing. “If you have a really wet silage, if you put up corn silage at 28 percent DM rather than 33 or 34 percent, or hay silage, you have a lot of water in that product and that’s going to dilute the acids that are being generated during the bacterial fermentation process,” said Van Saun. “That means it’s going to take longer to get the product fermented.”
Van Saun says soil contamination of ensiled feeds, such as from clostridial organisms that consume lactic acid, can be problematic. “Soil also breaks down protein,” said Van Saun. “That generates very high soluble protein, which is a real challenge from a formulation standpoint in getting the rumen properly balanced for nitrogen. It also generates butyric acid, which is modified by the rumen wall into a compound called beta-hydroxybutyrate, which is essentially ketones. You can induce ketosis through poorly-fermented silage.”
Hay crop silage can be much more challenging to adjust, and should be at a pH of 4.5 or less. The potential for listeria in haylage is higher when pH too high. “pH is a function of the maturity, moisture and sugar content of the plant,” said Van Saun. “Hay crop silage doesn’t have a lot of sugars, that’s why it’s a much more challenging product to ensile. When it’s taken out of the silo and used in the mix, it can potentially start to re-heat.”
Sensory evaluation of silage includes how it smells, looks and feels. A bunker face should appear smooth and straight, which limits the amount of oxygen that can enter the remaining silage. Van Saun explained if the face is irregular, the increased surface area means more area is exposed to oxygen resulting in a greater chance for microbial activity.
Silage color can also indicate quality problems. Silage with high butyrate levels will be slimy and green. “Bright yellow indicates high acetic acid content, probably a high yeast as well,” said Van Saun. “A lot of brown color and a tobacco or caramel smell indicates a burning reaction associated with heating.”
Van Saun says a silage pile will remain a fairly constant temperature if it’s good, stable silage. “A compost thermometer helps you get an idea of how deeply the oxygen is penetrating the pile,” he said. “The more the oxygen can go into the pile, the worse and more unstable it is.” Van Saun suggests placing a compost thermometer in at different depths of the silage to get an idea of oxygen penetration.
Silage odor is another good indicator of quality because it can be used to evaluate fermentation. High-quality, normal silage should have little or no ‘off’ odor. A vinegar odor indicates high acetic acid levels, while an alcohol odor indicates high ethanol.
Moisture levels in silage are important, but also the most subject to variability due to the many factors during harvest, transport and packing. It’s normal for moisture to vary throughout the pile, so it’s important to track moisture levels during feedout to determine dry matter intake. If silage is wetter than you think it is, the ration will contain less dry matter. If the silage is drier, you’re likely feeding more dry matter.
Particle size is critical to rumen function and cud chewing. Cow diets that have insufficient dietary fiber can lead to rumen acidosis and potential changes in milk composition. Particle size is best managed at harvest, so it’s important to use a particle separator to determine particle size and make appropriate adjustments. Particle size can potentially be altered during ration formulation and/or feed mixing, especially with overmixing or a mismatch between the feed ingredients and the mixing unit.
After mixing and at feeding, it’s important that particle size is consistent from one end of the bunk to the other. Cows are great at sorting, and will pick through a TMR and take what they like. Look for sorting behavior and determine whether sorting can be minimized by making formulation or mixing changes.
Although it’s too late to change particle size or packing density this season, being aware of the potential issues contained in a bunker full of one of the most important feedstuffs on the farm can help stop a nutrition issue before it starts.