Knowing how much a cow eats each day is the single-most important element in formulating cattle diets, but consistently and accurately measuring dry matter intake is a challenge. When livestock producers are asked how much the cows are eating, a common answer is often “all they want.”
Many factors affect how much feed a cow eats. It could be management, such as crowding at the feedbunk, cow comfort, time cows spend standing, feed quality, abrupt diet changes or water availability. It might be the weather: rainfall, temperature changes, humidity, wind chill and mud. Production factors and activity level also affect the amount cows will eat.
To make calculations more challenging, dry matter intake is a continuous variable. Just when we have determined the DMI for a particular time, something changes—such as cow movement, breeding, heat stress or forage moisture content—and the DMI changes, too.
Maintaining and monitoring dry matter intake are critical because cows have nutrient requirements that need to be covered to support their milk production and metabolic functions. Plus, feed cost is a principal expense, so producers don’t want to under-feed or over-feed cows.
Inaccurate moisture determinations on high-moisture feeds can also be a challenge. If using wet byproducts, haylage and corn, it is not uncommon to have 90 to 100 pounds of as-fed wet feeds in the diet. Moisture determinations that are 2 to 3 points off represent a couple pounds of dry matter difference. This can be significant.
Suppose that the ration has 50 percent corn silage, as fed, and we think it is 62 percent moisture, but it is actually 65 percent moisture. This means the cows will be eating less corn silage dry matter and more dry matter from the other forage and concentrate. If we are 2 pounds short on the corn silage dry matter, those 2 pounds would support somewhere around 4 pounds of milk production. Protein should not be as big a problem, but if the corn silage needs to be an effective fiber source, and we are short on effective fiber, this also might be troublesome for the percentage of fat and rumen function.
The more feed a cow can consume, the more milk she can produce, and maximizing the nutrient density improves the herd feed efficiency. A common benchmark is to have the average milk-to-feed ratio above 1.5 to 1. Fresh cows should have efficiencies of 1.7. Ratios below 1.5 could mean that feed intake is limiting and that improvements are likely.
Scales on feeding equipment must be accurate and checked and/or calibrated on a timely basis. Weighbacks of feed refusals are also critical in assessing nutrient intakes. Cows eat fairly predictable amounts of feed depending on their size and stage of lactation.
Neutral detergent fiber level and digestibility (NDFd) will have a profound impact on total intakes. Changes in NDFd can impact how much forage a cow is able to consume and digest every day. There can be significant differences in NDFd from one forage to another even if neutral detergent fiber and protein levels test similarly.
While computer modeling is sophisticated enough today to deliver very accurate calculations in support of a given amount of milk production, the models don’t always agree on predicted dry matter intakes. Errors in predicted and actual intakes make quite a difference in ration costs. Accuracy of dry matter intakes can also be improved with multiple production groups. These groups have smaller variation of milk production, so there is less variation in feed intake.
Bottom line, it is imperative to watch feed intake and milk-to-feed efficiency. The best strategy is to find the point where both are maximized.