In the past few years we have seen great advances in how we measure forage quality, especially when it comes to forage digestibility, which has a dramatic impact on feed quality and the resulting performance by the dairy herd.
We feed forages because of their carbohydrate content which, after fermentation in the rumen, provides energy for milk production. A large part of those carbohydrates are contained in the plant’s fiber. Commonly used measures of forage quality such as total digestible nutrients (TDN), acid detergent fiber (ADF) and neutral detergent fiber (NDF) only tell us about total fiber content and are not indicators of digestibility of the fiber. Two plants might look the same, have the same ADF and NDF but might differ on fiber digestibility and, as a result, give different results when it comes to overall milk production.
Feeding higher digestibility forages can help overall feed costs by potentially allowing us to feed less grains. Also, on the West Coast where we grow corn silage in hot weather, growing highly digestible alfalfa can offset the lower digestibility corn silage used in our typical rations.
Measuring Quality and Digestibility
One important measure of alfalfa digestibility is neutral detergent fiber digestibility at 30 hours inside the rumen (NDFD30). A newer measure is total tract NDF digestibility (TTNDFD), which is an indication of digestibility from zero hours to 240 hours of the feed inside the cow’s rumen. The 30-hour digestibility provides a window of time whereas the TTNDFD gives us a measure of digestibility the entire time the feed is in the rumen.
NDF digestibility is affected by the speed of maturity and the stage of maturity at harvest. Fast plant maturation has a negative impact on digestibility and obviously a more mature plant at harvest is less digestible than a younger plant. Factors that affect both speed of maturity and final maturity at harvest include plant dormancy, hot weather, irrigation schedule, other factors that might stress the plant, nutrient loss at harvest (raking, windrowing) and dirt contamination at harvest.
There are five factors we can manage to help us get closer to making the type of alfalfa we need in various rations for the dairy:
- Seed type and dormancy: Choose the right seed.
- Irrigation schedule: Size fields correctly.
- Time at harvest: Don’t rely on the calendar date.
- Nutrient loss: Most loss occurs between cutting and harvesting.
- Dirt contamination: Avoid excessive ash content.
With careful planning and training with the harvest team, dairy producers can get the right quality forage for each production group and maximize digestibility to get the most performance out of cows.