How do you calculate dry matter and water in animal nutrition in relation to cows?

Answer 1

Dry matter is the exact opposite of water or moisture content. In terms of feeds, you calculate the dry matter (DM) content by first knowing what feed you're looking at and estimating possible moisture content. For example, barley straw is about 15% moisture. So, 15% moisture minus 100 equals 85% dry matter.

However, the question is quite ambiguous making it more difficult to answer in a short stretch, so it really depends on if you're asking about how to calculate moisture/dry matter content of a feed fed to cattle (which was answered for you above), how much dry matter a cow eats (same with moisture content) in a day, or how much feed should be given to a cow in terms of as-fed (which includes moisture content) or on a dry-matter basis.

For the last two possibilities, either and both tie into one another. For instance, you cannot calculate how much a cow eats in terms of as-fed without first knowing what that cow is fed and how much she's going to eat, as an estimate. Ergo, you cannot calculate how much feed should be given to a cow in an as-fed basis without knowing what feed is being fed and how much she'll eat in a day. It is much easier to know how much a cow will eat per day on a DM basis only than it is how much feed should be given to a cow on an as-fed (or even dry-matter) basis, because you have to know the weight, age and type of cow being fed, and the quality and moisture content of feed fed.

That said (complicated, I know), we can look at an example to help you understand what's being said. Let's say you're wanting to feed a mature beef cow that weighs 1200 lbs good quality hay--legume-grass mix--and want to find out how much to feed (on an as-fed basis) in estimation of how much she'll eat in a day. On good quality hay, a cow should be expected to eat 2% of her body weight per day in terms of dry-matter content. The hay is estimated to have a moisture content of around 25%.

This means that a 1200 lb cow is expected to consume 24 lbs of hay (DM content, not as-fed!!) per day. To get that value, you go 1200 lbs x 0.02 = 24 lbs DM.

Next, knowing the moisture content of the hay, to find out how much that cow may eat per day on an as-fed basis, you calculate 24 ÷ 0.75 = 32 lbs per day AF.

Why did I use the 75% value instead of the 25% value, you ask? It's because the hay is marked at 25% moisture content, not dry-matter content. In order to get an as-fed value, we need to divide dry-matter by dry-matter to get as-fed. In other words, we need to use the value 100 - 25 = 75% Dry Matter instead of 25% moisture so that the dry-matter values can cancel each other out in order to get as-fed.

See, if we used the 25% value instead as a DM value, we'd get a HUGE number for as-fed: Like, 96 lbs of hay eaten per day huge. A cow doesn't eat that much hay in a day, realistically. Grass, yes, as she grazes in a pasture, but not hay. Hay has a much lower moisture content and even lower quality than grass does, which infers that cows will eat less of it.

A rule of thumb to remember when formulating rations for cows is that a cow will eat more of a feedstuff if it is a) high quality and b) has a high moisture content. As you begin to learn how to formulate rations for cows without using a computer to do the calculations for you, this is always a good rule to live by.