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Dry-Matter and As-Fed

Let’s go through an example using grass pasture.

Suppose we have a beef or dairy cow consuming 100 lbs of pasture per day. Is this a lot? We will answer this question shortly. Suppose we go out and clip 100 grams of this pasture as its growing in the field. We analyze it. We find that of this 100 gram sample, 80% is water, 13% are carbohydrates, 4% protein, 2% vitamins and minerals and 1% fat. Now remember the nutrients we are interested in, that would be fat, minerals, vitamins, protein and carbohydrates, would come in as an energy requirement. So, we are mainly interested in only about 20% of this feed on an as-fed basis because this is what is supplying the nutrients. Suppose we put that 100 gram sample in the oven overnight and dried it. Now we have 20 grams left because we lost 80 grams of water overnight. So now if we look at the nutrient concentration on a dry-matter basis, we find that this pasture contains 5% fat, 10% vitamins and minerals, 20% protein and 65% carbohydrates. How much dry-matter is this cow consuming per day? First, we have to determine the dry-matter percent of the pasture which is 20 grams, that is what it weighed after we had it in the oven overnight. Divided by 100 grams, which was the original sample weight. Multiply it by 100 to convert it to a percentage. And we arrive at the answer of 20% dry-matter. Secondly, we would then convert the as-fed lbs to dry-matter lbs. So the cows are consuming 100 lbs of pasture per day times .2 or the decimal form of 20% dry-matter and so we arrive at the answer of 20 lbs of dry-matter per day. Is this a lot? Not really for a lactating beef or dairy cow. Although on an as-fed basis this cow is consuming quite a bit of feed, 100 lbs, when we look at actually the nutrients she’s consuming, fat, vitamins, minerals, proteins, and carbs the animal is actually consuming a relatively low amount of dry-matter. It’s important to note the abbreviations that we will use throughout the remainder of the quarter, DM for dry-matter and AF for as-fed.

Let’s go through another example using our grass pasture and we are going to convert from dry-matter to as-fed.

Suppose we got an analysis back from the lab that said that our grass pasture contained 20% protein and 5% fat and our pasture sample was 20% dry-matter. On an as-fed basis, how much crude protein and fat is in the grass as its growing in the field? When you go from dry-matter to as-fed, we are adding back water, we are diluting the nutrients, that means the concentration of protein and fat must go down. So, if we convert our dry-matter percent to decimal form or 20 divided by 100, .2, times 20% protein should give us an answer of 4% protein on an as-fed basis. And if we do the same for fat, which is 20% dry-matter divided by 100, convert that to a decimal form of .2, times 5% fat, we should get 1% fat on an as-fed basis.

Here are some general observations when converting from dry-matter to as-fed or as-fed to dry-matter. The calculation will always include the nutrient or component, in the case of fiber or energy, and the dry-matter percent in the decimal form. You always have one of two options, multiply by percent dry-matter or divide by percent dry-matter. When converting from dry-matter to as-fed, the concentration is always less. Another way of thinking about this is dilution. The only difference between dry-matter and as-fed is water. What happens when you add water to the feed? Everything gets diluted, therefore the concentration decreases. So when converting from dry-matter to as-fed, the concentration of the nutrient or component always decreases. Now, when converting from as-fed to dry-matter, the concentration is always greater. Again, the only difference between as-fed and dry-matter is water. Another way of thinking of this is concentrating something by taking the water out. So what happens when you dry feed in the oven or take the water out? Nutrients get concentrated and then therefore concentration increases. So when converting from as-fed to dry-matter, concentration always increases.

Next, we will go through the example again to try to emphasize the importance of dry-matter to as-fed or as-fed to dry-matter conversions. Suppose we have a pen of 100 dairy cows and we are delivering five tons of feed to this pen daily. We want to know how many lbs of dry-matter, on average, is each cow in the pen consuming. If we are delivering five tons, there are 2,000 lbs per ton, so that means we are delivering 10,000 lbs of as-fed feed per day. We know that the diet is 55% dry-matter. If we divide 55 by 100, to convert it to a decimal form, multiply it by the 10,000 lbs that we are delivering, that means we are offering the animals 5,500 lbs of dry-matter per day. If we have 100 cows in the pen, 5,500 lbs of dry-matter divided by 100 cows equals 55 lbs of dry-matter per cow per day. So that would be our average consumption.

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