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The cow’s energy budget

Lynn Jaynes Published on 11 September 2014

cow working at a desk

Let’s say you’re in college, your tuition is paid, and you have $1,000 a month to use for living expenses (You wish, right?). What will you do with it? Well, let’s see – you have housing ($400); gas ($120); pizza, instant noodles and an occasional hot dog ($130); books, school supplies and parking fees ($300).



That leaves you $50 to spend on miscellaneous necessities (the dollar movie theater, breath freshener for your date), and that equals $1,000. Easy-peasy. It works.

But then the fuel pump goes out on your car ($150). Son-of-a-gun. You can rob some funds from the gas budget, because your car isn’t going anywhere for a while anyway, but where does the rest come from? You have two choices: increase your income or reallocate what you have.

You also have a bigger problem than the fuel pump: You forgot to allow for the percentage you’re going to lose in taxes off the top. And now you’re in a negative budget situation.

A dairy cow’s energy is on a budget, too. It’s not nearly as exciting as deciding whether to eat pizza or instant noodles, but it’s the same principle and it’s still challenging. A cow has several places she can spend her energy. Let’s start her dietary budget with 60 percent forage and 40 percent concentrate.

While on the surface it looks like she should be able to spend the whole budget, in reality she can only spend about 70 percent of it (the digestible portion), because she’s going to lose 30 percent in “taxes” – or the non-digestible portion expressed in urine, feces and gas. However, similar to the college budget, we can help her out by increasing her energy income or by reallocating her energy expenditures.


Allocating the budget
We can’t help the cow unless we first understand her budget. A cow’s energy budget has two basic sources – carbohydrates (fiber and non-fiber components) and fats/oils. There is a third source, protein, but that’s generally used as a last resort because it’s expensive and very energy-inefficient as an energy source as needed to excrete the nitrogen in the urine.

Her gross energy or “beginning budget” from feed sources will look like this:

Gross energy in feed: 100 percent
(lost 30 percent to feces right off the top)

Digestible energy: 70 percent
(5 percent lost in urine and 5 percent lost in gas)

Metabolizable energy: 60 percent
(lost another 20 percent due to rumen function, as heat during fermentation and nutrient metabolism or microbial growth; after all, it takes energy to spend a budget)

Net energy: 40 percent


Her net energy is what she has left to spend.

Dietary energy is usually measured in megacalories (Mcal), and we’ll use the Mcal approach to quantify her budget in this example.

Of her available budget (40 percent as expressed above), the first energy expenditure for any cow is maintenance. She’s going to live, breathe, move around, digest feed and replace worn-out tissue with about 10 Mcal per day. The energy budget will take another hit with energy used for growth.

A rule for first-lactation cows is 20 percent above maintenance, and second-lactation cows will use 10 percent above maintenance. So if we’re using 10 Mcals per day for maintenance, we’ll use another 2 Mcals per day for growth in a first-lactation cow until she reaches a mature size.

A third energy budget item is reproduction. This requirement increases gradually until the last month or two of pregnancy when as much as 3 Mcals per day are required to grow the fetus (assuming a single birth, not twins).

Another energy budget item is weight gain. Cows can gain weight much like humans do, and 2.33 Mcals are used per 1 pound of weight gain. Conversely, if she loses weight, she’ll give up a similar amount of energy for other functions.

Since your dairy cow is bred specifically to produce milk, a significant portion of her energy balance will be used in milk yield. It takes about 0.31 Mcals to produce 1 pound of milk containing 3.5 percent milk fat. So if she’s producing 70 pounds of milk, she’ll need 21.7 Mcals to maintain this yield.

The early lactation budget
The question to investigate is how your cow is spending her energy budget. Let’s assume she’s in the early lactation period. If she has a 35-Mcal- per-day budget to spend, she’ll likely allocate it like this:

Maintenance: 10 Mcal per day

Milk yield & milk fat: 21.7 Mcal (because she’s been bred for production)

And now, assuming she’s receiving 35 Mcal per day, she only has 3.3 Mcal left to spend on weight gain, growth and reproduction. And when she runs out of energy after the first two expenditures (maintenance and milk yield), she may not gain weight, may not cycle and may not grow.

The late-lactation budget
A late-lactation cow will spend her budget a little differently. First, she still wants to stay alive, but as she becomes pregnant, her energy budget will shift to protect the fetus, so she’ll give up other functions to maintain the pregnancy.

After that will come growth or milk yield and then finally weight gain. So if a late-lactation cow is thin, we’ll have to help her with the energy requirements to maintain all other energy costs before we can expect her to produce well or gain weight.

Can we manipulate the energy portioning or change the order of how those energy bills are paid? Somewhat. There is an energy-partitioning agent known as bovine somatotropin (BST), which will change the rules a little bit and drive the cow toward more milk production.

But a key concern when using BST is to have adequate nutrients to meet the extra 6 to 10 pounds of milk the cow will be producing. The energy still has to come from somewhere.

Increasing the budget
To increase the cow’s energy budget, one of the first things we can do is look at the “taxes” or the non-digestible energy. Anything we can do to make the non-digestible portion more digestible is a winner (Who wants to pay taxes, right?). We can do that by feeding higher-quality forage or through better forage processing.

Let’s go back to the college budget analogy. If we had money stashed away in a lock-box, it would be easier to access it if we had a key or the lock-box was made out of something we could break open, like wood.

Plants have the same issue – we either need to “unlock” the energy or figure out a way to disintegrate the covering. Plants have two levels of “coverings” in NDF (neutral-detergent fiber) and ADF (acid-detergent fiber). NDF is essentially the cell wall, including ADF and hemicellulose. The good news is: Hemicellulose can be about 60 to 70 percent digestible, but it increases as the plant matures.

ADF is composed of cellulose and lignin. Its digestibility is modestly low, typically 30 to 40 percent. ADF will also increase as a plant matures, causing plants to become woody or straw-like. High ADF will equal low energy content.

So anything we can do to harvest forage at lower maturity will increase the availability of the energy (unlock the box) and increase the cow’s energy budget. As the plant matures, it essentially “locks” the box and makes the energy less available.

Another way to increase the energy budget is to use additives. We can take that 5 percent we’ll lose in urine and another 5 percent lost in gas production and use monensin (Rumensin) to reduce that loss, resulting in more energy in the budget.

Balancing the account
There are different ways to express energy content in the diet. The traditional way was called TDN (total digestible nutrients). A second commonly used approach is to discuss “net energy,” as we’ve done here. But the key things to remember, no matter how it’s discussed, are:

  • Cows will partition energy based on stage of lactation and reproductive status.
  • As the fiber and ash levels increase in forages, the level of available energy in the diet goes down.
  • If you have fats and oils in the diet, you’ll have more energy.
  • There are several sources of fiber in the diet with varying amounts of availability.
  • Cows can’t produce excellent milk yields, a calf and be expected to grow unless you understand her energy budget and are actively managing it.

And last, you’re going to do better in college if you eat something other than instant noodles. Just sayin’. PD

Many universities with dairy programs also offer easy-to-follow presentations on CD-roms that you can watch at your leisure (check one out at the University of Illinois Extension's website, Pubs Plus).

Illustration by Corey Lewis.

lynn jaynes

Lynn Jaynes
Progressive Dairyman