The author is president of the William H. Miner Agricultural Research Institute, Chazy, N.Y.
The relationship between forage neutral detergent fiber (NDF) digestibility and dairy cow performance is well known. For each one percentage-unit rise in NDF digestibility, we expect to observe 0.4 pound per day more dry matter intake and about 0.55 pound per day greater 4% fat-corrected milk. This robust relationship holds true over a wide range of forages commonly fed on dairy farms.
If we focus specifically on higher corn silage rations, which many producers feed, we see a similar relationship. For diets containing more than 40% corn silage, each one-percentage unit increase in NDF digestibility boosts both intake and fat-corrected milk by about 0.3 pound per day.
When we think of high-quality forage, we almost always think of lower fiber content and greater fiber digestibility. In other words, we think about forage quality from the perspective of the high-producing dairy cow. But, of course, we are feeding a variety of cows, from early to late lactation, dry cows, close-up cows, and replacements.
Match forage to cows
Knowing that higher NDF digestibility enhances feed intake and milk production is only part of the story. We also need to understand that not all cows will respond similarly to higher-quality forage. So, we need to target forages, based on its quality, to the groups of cows that will respond most effectively.
Larry Chase from Cornell University refers to “right-quality forage” versus “high-quality forage.” I already mentioned that we usually think of high-quality forage as low NDF content, high NDF digestibility, and overall higher carbohydrate fermentability. But Chase’s point is that this forage may not be high quality for later lactation cows or dry cows.
A good example of the variable response to forage quality was shown by Nebraska researchers who evaluated the milk response to a high versus low NDF digestibility corn silage over a wide range of days in milk and milk production. For cows ranging between 30 and 122 pounds per day in milk production, the average response to the higher fiber digestibility corn silage was only 2 pounds per cow per day. That is not much milk response, on average, but how did individual cows respond to enhanced fiber digestibility as they varied in milk production and stage of lactation?
Individual cows ranged from no response (even a negative response) to a highly positive milk response. In fact, cows producing greater than about 70 pounds per day had an average milk response of about 15 pounds per cow per day to the greater NDF digestibility corn silage. Similar variability in milk response among individual cows has been shown for variable forage-to-concentrate ratios, grass and legume forages, brown midrib versus conventional corn silage, and starch fermentability.
In contrast, lower-producing cows either did not respond or even responded negatively in milk production to greater forage NDF digestibility. Some lower-producing cows lost up to 10 pounds per day in milk production when they consumed the higher NDF digestibility corn silage. This negative milk response is explained by what we now know about how cows respond physiologically to diet carbohydrate fermentability.
The bottom line is that feeding dairy cows high-quality forage with high-fiber digestibility typically enhances feed intake and milk production, but we must consider the milk production level of the cow to feed these forages most efficiently. Feeding high-quality forage to lower-producing cows often squanders its feeding value. Feed the right quality forage to the right group of cows!
Housing cows in homogeneous groups and targeting the right quality forage based on stage of lactation can pay large dividends in milk production and component output. Some farmers feed a single total mixed ration (TMR) to all lactating cows because they want to avoid any reduction in milk yield that may occur when cows change diets or switch groups. However, research at Michigan State University discovered the importance of dietary NDF and starch content and digestibility to minimize reductions in milk yield when moving cows from group to group.
Current recommendations are to group cows and feed them such that early lactation, high-producing cows receive diets of high fermentability containing highly digestible forage NDF and starch. Lower-producing cows need diets of lower fermentability. They still require digestible NDF but with lower rumen fermentable starch. Feeding diets high in fermentable starch to later lactation cows reduces feed intake and milk yield and elevates body condition score.
A study conducted at Miner Institute compared brown midrib corn silage to mixed, mostly grass hay crop silage in a similar approach as the previously mentioned Nebraska study. The lower-producing cows responded better to the hay crop silage whereas the higher-producing cows had a better milk response when fed the corn silage.
We know that cows respond differently to the digestibility or fermentability of their diet, depending on their stage of lactation. We also know that fiber digestibility and its rumen-filling effect play a key role. Work with your nutritionist to be sure you are feeding forage with the right fiber and fiber digestibility to the right groups of cows to capture the most value from your forage crops.