May 11, 2016 
While hay is just dehydrated grass there are several differences between hay and the live plant that you need to take into consideration when adding pasture to the horse’s diet.
Note: References in the following discussion to calories, protein and amount consumed are on a dry matter basis; weight of the pasture or hay after water weight is subtracted.
The first consideration is calories. When grass is cut to make hay it will continue to draw on its carbohydrate stores and metabolize them until moisture level drops low enough to stop enzyme activity. How long this takes depends on the curing conditions. Hays cured under very hot and dry conditions rapidly drop their moisture level and preserve carbohydrates, and therefore calories, with low losses. The longer the curing takes, the more calories/carbohydrates are lost.
Calories are expressed as digestible energy. The range for pastures is from 1.78 to 2.74 Mcal per kg which corresponds to the levels found in moderate quality grass hay to that in beet pulp with molasses. Younger growths of grass will have the higher calorie levels. Average DE for pasture is 2.26 Mcal/kg.
Protein levels in grass also vary widely, from 7.5 to 22.7% crude protein with a mean of 15.2%. Again, it is the younger growth stages that have the highest protein with even grasses reaching into the 20s. By the time grass has grown and matured enough to be cut for hay it is generally in the 8 to 12% protein range.
Live pasture contains the full range of vitamins except vitamin D which the horse will manufacture in the skin from sun exposure. When cut for hay, vitamin C and E and the B vitamins drop. Vitamin A is also lost but more slowly and levels remain adequate in most hays with a good green color.
On the mineral front, grass pastures may not have adequate calcium for lactation or growth (although mixed legume/grass will) and are often but not always deficient in phosphorus for adult horses. Sodium and chloride (salt), as well as copper and zinc are generally deficient. This pattern is the same as in hays.
A common question is how to adjust the diet when a horse begins to be turned out on grass. How do you estimate how much grass the horse is eating. Because of the higher energy/calorie level of grass, this is more than a matter of curiosity. Left to their own devices, a horse consuming 2.5% of its body weight of pasture with an average energy level of 2.26 Mcal/kg would gain a full body condition score in 30 days. Even shorter turnout periods result in a jump in calories that should be taken into account to avoid weight gain.
Estimating can be easier said than done. Research has demonstrated that horses turned out for short periods of time will increase the amount of grass they consume to compensate for the short exposure. Grass intake on a dry matter basis can be estimated using the equation:
g DM per kg of body weight = 5.12√x – 2.86 (Paul Siciliano, North Carolina State University
where x = hours grazing with unlimited grass. To illustrate, for 4 hours grazing intake would be 5.12 x 2 = 10.24 – 2.86 = 7.38 g, 3.69 kg for a 500 kg horse . A high quality grass hay will have a DE of around 1.9 Mcal/kg, while average pasture is 2.26 or 1.2 times greater so the 3.69 kg of grass = 1.2 x 3.69 = 4.43 kg of hay. For very young pastures this will be an underestimate and for fading pastures may be an overestimate but it gives you a good working point. Subtract that from your horse’s usual hay ration. [Note: Hay runs about 90% dry matter, 10% moisture, so 4.43 kg of hay DM = 4.43/0.9 = 4.9 kg as fed.]
This equation works for ponies and full size horses. It can be applied to turnout times between 3 and 24 hours.
Grazing your horse has clear nutritional benefits and can save feed dollars. Manage your feed adjustments to avoid weight gain by calculating estimated pasture intake.
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