When it comes to growing the right forage for the cow, we need to understand how the cow utilizes forage. Ultimately, she needs to get calories out of the forage that we feed her. Generally speaking, this comes in the form of digestible fiber and digestible starch.
Digestible starch is fairly straightforward; corn silage with a higher starch content will have more calories. It is also related to how digestible the starch is (starch kd rate). The longer corn silage is in storage, the higher its starch kd rate or the more digestible it is.
How does the forage portion work? Here we need to evaluate the units of digestible fiber a cow can get from a forage in the first hour after she consumed it. To do this, we need to know how much neutral detergent fiber (NDF) undigested NDF 240 is in the forage, as well as the NDF kd rate. The uNDF 240 is the amount of fiber that is undigestible after 240 hours, and the NDF kd rate is the rate of fiber digestion per hour. When we take the pool of potentially digestible fiber (NDF minus uNDF 240) and multiply it by the NDF kd rate, we get the total units of NDF that a cow can digest in the first hour after consuming it. From a cow’s perspective, the more NDF it can digest per hour, the more calories that forage provides for milk production.
How do the cow’s needs translate to various forages?
Let’s look at corn silage first. When we compare conventional corn silage to brown midrib (BMR) corn silage, what is the difference? The fiber in BMR corn silage is more digestible than that of conventional corn silage. That is due to less uNDF 240 in BMR compared to conventional corn silage, and the NDF kd rate is slightly faster in BMR versus conventional. In other words, BMR corn silage has a larger pool of potentially digestible fiber and, thus, cows can digest more units of NDF per hour from a BMR compared to a conventional corn silage.
Similar rules apply when we compare alfalfa with Italian ryegrass. Alfalfa and Italian ryegrass have similar NDF values, but Italian ryegrass has a lower uNDF 240. The NDF kd rate of Italian ryegrass is slower than alfalfa, but because its uNDF 240 content is lower, Italian ryegrass ultimately produces more units of NDF digestion in the first hour compared to alfalfa due to having a larger pool of digestible fiber. This creates more calories available to the cow from fiber digestion. For comparison’s sake, Italian ryegrass and straw have a similar pool of potentially digestible fiber. However, because straw is so high in NDF and has a very slow rate of digestion, straw provides a ton of bulk to the diet and far fewer calories compared to Italian ryegrass or alfalfa.
How do the cow perspective and forage perspective ultimately impact your farm’s nutrition program?
Farms are faced with higher fertilizer and seed prices, as well as unpredictable weather. Amid these challenges, the goal remains to grow the best-quality forage to meet cows’ requirements. Commodities are also volatile, and, as we enter a new growing season, we need to consider how to maximize digestible tons per acre relative to commodity prices.
The above discussions do not conclude that BMR is better than conventional corn silage or that Italian ryegrass is better than alfalfa. Rather, farms need to consider the differences in pools of digestible fiber among various forages. If we continue with the mindset of growing the most tons per acre, are we really growing the forages that provide the best opportunity to improve margins? We need to consider how to grow the most digestible tons of fiber per acre, as well as digestible starch, at the most economical cost.
To accomplish this goal, we need to factor in commodity prices as well as fertilizer costs. Start by looking at your current rations and your current cost per hundredweight of energy- or revenue-corrected milk. As you begin, determine how many tons of digestible fiber you are currently feeding per day and then determine the various crops you need to grow to produce those tons. In addition, you can select varieties of alfalfa, grass or corn that provide bigger pools of digestible fiber or starch compared to other varieties. In addition to varieties, planting at correct population densities (when it comes to corn silage) to achieve maximum fiber digestibly needs to be considered. Also, the more uNDF you provide the cows, the more manure you must haul back to the field.
As we continue to look for ways to add margin to our operations, we need to change the perspective of growing forages by starting with the cow’s perspective to maximize profit potential. Harvesting forage to get maximum tonnage per acre isn’t always what the cow wants or needs. Evaluating varieties based on potential tons of digestible fiber or starch produced per acre can help you make the most economical decisions for your farm. Beginning with the cow and what she needs changes our forage production strategy.
PHOTO: Lynn Jaynes.