It’s probably not news to you that horses are herbivores with digestive systems designed for grazing around 18 hours a day. Or that the foundation of an ideal equine diet is high-fiber forage, such as pasture and hay. But making practical use of that knowledge can seem complicated. Thankfully, it’s not all that difficult to do.

But first, here’s a quick refresher on why feeding your horse enough hay is important to his health and well-being:

• it provides the fiber necessary for optimal digestive system function, reducing the risk of colic and other gastrointestinal problems.
•  is a good source of energy, protein, vitamins and minerals at varying levels, depending on forage species.
• is a good source of energy, protein, vitamins and minerals at varying levels, depending on forage species.
• encourages healthy hindgut microflora vital to fermentation and digestion.
• consumption increases the horse’s chewing rate, reducing the risk of cribbing and other stereotypic behaviors. Increased chewing also encourages saliva production, which in turn helps to buffer the acid produced in the stomach, reducing the risk gastric ulcers.

More hay can’t hurt

Despite these benefits, many owners worry that feeding too much hay can adversely affect performance. But research has shown that this is not true; in fact, studies suggest that high-energy fibrous feeds may even adequately support elevated nutritional requirements.

Another worry is that hay has potential to increase a horse’s bodyweight due to the water-absorbing capacity of fiber. However, strategic feeding and a short period of dietary restriction prior to competition can mitigate that possibility.

So how much hay does your horse need? If he is kept on pasture and offered it free-choice when grass isn’t growing, he’s going to consume enough forage on his own. For horses stabled even part of the day with limited access to pasture, however, you’ll want to calculate the minimum and adjust from there.

Before we begin, it’s important to understand two terms: “as fed basis,” which means the actual amount of hay you give the horse by weight (more about that later); and “dry matter” (DM), which is everything in hay except the water—the protein, carbohydrates, minerals, etc.—and defines the nutrient value of the forage. Ideally all hay would be sold with a basic analysis including DM. However, in practice, few horse owners receive a detailed analysis of the hay they purchase so we estimate that most hay is 90 percent DM. Haylage has a lower DM percentage because it contains more water, but it is not commonly fed to horses in the United States, so we won’t dwell on that.

Click here to learn more about ration balancers.

General guidelines to consider when calculating hay rations:

• Horses at a healthy weight are ideally fed a minimum of 1.5 percent of their body weight in dry matter (DM) per day. If the hay is of good nutritional value, some horses can do just fine on this ration alone, with no supplementation with grain.

• Horses in high intensity training need a minimum of 1.25 of their body weight in DM as hay. The remainder of the ration would be supplied as a grain concentrate.

• A weight-loss regimen for horses would provide less than 1 percent of their body weight in DM as forage. A ration balancer can be a good way to ensure that overweight horses get all the necessary nutrients with limited hay and grain. Likewise, a “slow feeder,” which extends the time it takes to eat a hay ration, can be beneficial because it preserves many of the physical and mental benefits of a hay-based diet.

To calculate your horse’s minimum forage requirements, follow these three steps:

1. Estimate your horse’s body weight. There are several ways to do this, including using a weight tape or livestock scale.

2. Determine the minimum DM forage requirement of your horse, according to his body weight and workload.

3. Calculate the minimum amount of forage your horse needs for optimal health. To convert the amount on a dry matter basis to the amount as fed, divide the amount of forage on a dry matter basis by the dry matter percentage of the feedstuff.

​For example, what is the minimum amount of hay that an 1,100-pound mare in good condition and medium work would need? Here are the steps:

1. 1,100-pounds

2. Calculate 1.5 percent of 1,100 to determine DM forage requirement. (1,100 x .015 = 16.5)

3. Divide that DM requirement by 90 percent (the presumed DM in the hay unless otherwise known) to determine how much hay to feed. (16.5 /.90 = 18.33)

So this mare would need a minimum of 18.33 pounds of hay per day. What if she were in high-intensity work, such as upper-level eventing or racing? Then she would need 1.25 percent of her body weight, or 13.75 on a dry matter basis, which is 15.27 pounds of hay. Remember that a horse in higher intensity work needs more calories, so his minimum hay requirement will be lower but the balance of his diet will be rounded out with a higher percentage of high-calorie grain/concentrates.

Weighing hay is a critical part of this process. However, I know that in practice hay is often fed by the flake rather than the pound. This often leads to imprecision. For example, flakes of grass hay generally weigh less than legume-based flakes of similar size. Even bales of the same type of hay can have different weights. Leaves generally pack better than stems, so as plants mature and develop a higher proportion of stems to leaves, bales generally become less dense and lighter.

Weighing hay is easy

You can do this by stepping on a scale yourself and noting your weight. Then step back on the scale while holding the hay and do the math. Or hang a fishing scale from the rafters of your hay storage area and weigh hay in a net.

Resist the temptation to simply “eyeball” hay amounts. If you aren’t going to feed it free-choice—giving your horse more than he’s going to eat before the next portion arrives—weighing his hay is absolutely necessary to ensure he’s getting enough. I realize many people don’t do this, but it’s the only way to be certain you’ve got it right.

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