It’s no secret that providing enough energy in a high-producing dairy cow’s diet can be a challenge. In fact for all animal agriculture – whether it be dairy cows, beef cows, goats or horses – the most limiting nutrient for optimal performance is energy. The bovine, equine and caprine species are herbivores – animals that must consume forages in order for their complex digestive systems to function properly. However, forages – grasses, legumes and silages – do not provide the necessary calories (energy) that our highly productive animals require to produce optimal meat, milk and fiber.
All feedstuffs that provide any level of nutrition will contain carbohydrates, forages included. But the physical nature of forages make them less digestible, providing less energy in diets. Feedstuffs that contain larger portions of Non-Fiber Carbohydrates (NFC) have been used in animal agriculture for many years to add more energy to animal diets. As the term NFC suggests, these feeds contain carbohydrates that are not associated with complex fibers such as cellulose and lignin. Non-fiber carbohydrates contain higher levels of sugar and starch – all of which provide more calories in an animal diet.
With one of the highest energy densities of all feedstuffs fed to animals, starch is the leading energy-producing carbohydrate used in dairy cow diets today. As the feedstuff that provides the most starch in U.S. dairy diets, corn is king in the animal feed industry. Ruminant scientists discovered many years ago that even though cow stomachs require high amounts of forages, they can also tolerate and perform very well with more rapidly fermenting carbohydrates such as starches and sugars.
While starch is found in other grains such as oats, barley, milo and wheat, the vast majority of research conducted on starch digestion in dairy cow diets has been done with corn due to the dominance of corn in worldwide agriculture. Corn provides more calories per acre than any other crop. In spite of a recent increase in prices, corn is still the energy source of choice in most dairy cow diets.
Optimal milk production for dairy cows and growth rates for heifers is both a function of proper energy metabolism and protein metabolism. Optimal energy metabolism is the result of carbohydrates being converted to glucose in the blood stream of all animals. The more blood-glucose that a cow can synthesize, the more milk she will produce and the more rapidly a heifer or calf will grow. All feeds that are digested in a rumen will produce energy. However, carbohydrates such as starch ferment more rapidly in the rumen and supply more total energy.
As a result, the dairy industry has embraced feeding corn and corn byproducts such as distillers grains, corn gluten feeds and hominy because supplies of those products are plentiful and relatively inexpensive. Corn silage has become the forage of choice for much of the nation’s dairy and beef industry because it is easily grown in most parts of the country and provides more calories per pound of dry matter compared to grasses, pastures and hay crop forages.
But because cows are still herbivores requiring nearly half of their diet to be fiber-based feedstuffs, the total amount of starch that can be fed to cows, calves and heifers is limited. Rumen health and cow health is negatively affected by excessive levels of starch and total NFC in rations. Over-feeding starch to cows will quickly result in a hostile environment for rumen microbes. All-grain diets fed to herbivores will kill them.
The fermentation of starch results in a lower pH level in the rumen and fiber-digesting microbes cannot survive in overly acidic conditions. The general term used to describe this problem is “ruminal acidosis”. Signs of excessive starch levels and ruminal acidosis in a dairy diet include declining milk fat percentage, declining cud-chewing activity and declining milk production. Long-term effects include laminitis and an increased incidence of ketosis and displaced abomasums.
The optimal starch level for dairy cow diets is not well established because diets can have great variation in effective fiber as well as fiber digestibility. However, most nutritionists agree that total starch on a dry matter basis should fall between 25 and 30 percent of the total diet. The proper balancing and managing of starch and total NFC levels in dairy cow diets is intended to maximize milk production while minimizing deviations of ruminal pH throughout the day. Low rumen pH damages the rumen papillae, reducing the absorptive surface area in the rumen, which results in reduced feed digestion. Milk production is ultimately driven by feed intakes so when feed absorption and digestibility is compromised, milk production will suffer as will milk components and cow health.
For most dairy farmers, corn silage and corn grain supply the majority of the necessary energy in dairy cow diets. Ear-to-stalk ratios and the size of the ears will have a large influence on the amount of starch found in a silo of corn silage. The starch level in corn silage is a major contributor to the calories that it can supply to a milk cow diet.
More recently, as corn prices have soared due to drought and market demands, there has been a greater interest in the digestibility of starch in corn silage. Farmers who can grow high quality corn silage with superior starch digestibility have been able to reduce their dependence on higher priced purchased corn. But corn silage must be grown, harvested and stored correctly to get the most out of the starch. We’ve all seen the undigested corn kernels in the manure, indicating that many calories that could have been used for milk production have been wasted.
Digestibility of starch in corn silage is highly variable and affected by the growing environment and the maturity at harvest. Corn kernels harden as the corn plant matures and becomes drier and less digestible. Corn kernels that are beyond the milk stage or no longer have any sign of soft dough will supply little nutrition to the ration even though the “forage test” says there’s a lot of starch and energy available.
The addition of starch in today’s dairy herd diets is a sure bet to improve milk production or growth rates. However, when formulating rations that are especially high in corn silage and/or corn grain, make sure that starch is not overfed or money is not being thrown away with poorly digestible starch.