Forage is an indispensable part of dairy cow diets in maximizing milk production and maintaining rumen health. However, due to its bulkiness and the inherently low levels of fiber digestibility, cows are limited in how much forage they can consume every day and still keep feed dry matter intakes at high enough levels to support high milk production.
Even though cows are biologically designed to digest forages, their improved genetic potential requires that they digest far more feed and nutrients than an all-forage diet can supply. Getting cows to milk on high forage diets requires keeping the forage quality high and consistent and making it available throughout the year.
Attempting to maximize forage in diets and still maintain high levels of milk production can be challenging. At the heart of the forage limitation is the amount and digestibility of the fiber fraction of the forage. The fibrous fraction of all forages is referred to as neutral detergent fiber (NDF), derived from the laboratory procedure developed to analyze fiber.
Determining forage needs
The amount of forage a cow can consume is almost totally dependent on the NDF level of that forage. Forages that test high in protein usually have low levels of NDF. Protein and NDF tend to be inversely proportional. Several studies have determined that daily NDF intake from forages for dairy cows is limited to about 1.1 percent to 1.2 percent of their body weight (BW). As an example, using the maximum value of 1.2 percent of BW for a cow weighing 1,500 pounds, her maximum NDF intake from forage would be 18 pounds.
If the same cow is fed a grass hay crop testing 60 percent NDF, the maximum 18 pounds of forage NDF would be reached when she consumed 30 pounds of the hay (18/0.6=30). If we expect this cow to produce 100 pounds of milk, her total dry matter intake should be near 60 pounds per day. The 30 pounds of forage from hay will only make up one-half of her diet dry matter intake. The remainder of the diet would need to be grains, concentrates or byproducts.
If, on the other hand, the NDF of the grass hay crop tested 50 percent, the cow could consume 36 pounds of forage (18/0.5=36). That’s an additional 6 pounds of forage the cow can eat, which brings the total up to 60 percent of the diet and reduces the need for other purchased feeds. The only way to feed higher levels of forage is for those forages to be lower in total NDF.
As forages grow, mature and increase in size, stems must become thicker and stronger for the plant to support itself. Stem material undergoes a process called lignification as the cellulose and protein weaves into tighter matrices to become more rigid. Lignin is nearly indigestible in ruminants and other forage-consuming species and the more lignified a plant becomes, the less digestible it is. Higher levels of NDF usually correspond to higher levels of lignin.
Forages in the early vegetative stage have larger leaf-to-stem ratios and lower levels of NDF. Immature forages can have crude protein levels over 20 percent on a dry matter basis and sugar levels over 10 percent. These nutrients feed the bacteria and protozoa that are necessary for the fermentation process in the rumen.
Significance of forage quality
Forage quality is highly variable from field to field, cutting to cutting and season to season. Rarely can a farmer get hay off the same field to test the same twice in a row. Changes in forage quality – either for the good or for the bad – will have a significant impact on both milk production and milk components for dairy herds. Dairy farmers who grow their own forages should focus on a diversity of forages.
Throughout the eastern U.S., perennial grasses such as orchardgrass, timothy, canarygrass and fescue are commonly used for animal feeds. Alfalfa is also being grown in many areas of the northeast. Many farmers are now double cropping with small grains such as winter rye along with triticale – a wheat/rye hybrid. All of these forages can be either baled or ensiled. Each one is an excellent feed when harvested in the early vegetative stage. As soon as they bloom or produce seed, however, the nutritional quality quickly diminishes. Timing is everything when it comes to harvesting forages.
When growing hay crops for dairy cows, there’s always the tradeoff of quality to tonnage. Farmers are inclined to maximize yield per acre. Tonnage and high yields, of course, equate to plant maturity and plant maturity equates to lower rumen digestibility and less metabolizable nutrition. Dairy farmers shouldn’t be focusing on how many tons of feed they get off a field – they should be focusing on gallons of milk per acre. For grass hay crops, maintaining an NDF level between 50 percent and 60 percent seems to be a good compromise between tonnage per acre and milk production.
This is a key concept that must be well understood when making the most out of raising your own forages. Unless aggressively managed, home-grown forage can quickly become heifer and dry cow hay.
As tempting as it is to grow your own feed to offset the rising cost of grains and byproducts, forages are completely unforgiving when it comes to low quality, and the drop in milk production will more than offset any savings on purchased feeds.
With a renewed interest in the economic value of feeding forages to dairy cows, researchers and nutritionists recognize the importance of being able to more accurately predict how any given forage will digest and what level of nutrition can be expected from it. And while the cows ultimately will tell the story of whether the forage they consume is good or bad, nutritionists are helpful in determining beforehand how forage will behave in the digestive system before a dairy farmer puts it in front of the cows.
Dairy farmers who focus on feeding forages that are low in NDF and cultivars that have been proven to be more digestible will be able to feed more forage and keep the grain bill down. Higher forage diets usually mean healthier and more profitable cows.