Pasture is the delight of horses – they run, buck, play and sleep in pasture, in addition to eating it. Because pasture serves so many purposes, it's important to avoid wasting this valuable product. If managed properly, pastures can provide much of a horse's nutritional requirements at a relatively low cost.
But overgrazing will cause pasture damage that, while solving a feed shortage in the short term, will cost more in the long run. In addition, a poorly kept pasture can be a source of internal parasites.
Pasture varies greatly throughout the world, but some generalizations can be made. First and most obvious is that the dry matter is only 20 to 30 percent of the total weight, compared to over 90 percent for dried hay or grain. Horses get a lot of water (and it's obvious at times!) from grass. They need to graze up to 60 percent of the day and night to consume the same amount of dry matter, which is three to four times as much by weight.
Pasture is composed of a variety of grasses and weeds. Many types grow from full dormant state to full maturity in 12 weeks, with a rapid phase in the beginning (the first 4 weeks), causing a one-half to two-thirds increase in height.
Young grasses contain the most digestible energy (0.9 to 1.15 Mcal/lb) and protein (9-21 percent), the stuff that makes your horses get fat. The only problem is that young grasses are short, and they get mowed down in no time at all.
Overgrazing at this stage ruins pastures. Be patient. If you want to stretch your pastures, avoid turning your horses out into those early growing pastures for any length of time.
Ideal Grazing Time
The ideal grazing time coincides with the period when the plant is sufficiently mature to be harvested as hay. This is the time when the grass contains sufficient nutrients (not too mature), yet there is a large mass of green stuff (not too early). But nobody waits that long unless they have plenty of pastures that they can rotate through.
If you wait a few weeks to turn out your horse onto lush pastures, you need to avoid overfeeding. Understandably, this will require some coaxing to return the horse to the indoor, or grassless paddock. For example, you could turn out horses onto fresh pasture for no more than one hour per day for the first week, then add one hour per week thereafter.
Over the spring and early summer the grass will mature, drying to 40 to 50 percent dry matter, decrease in protein to 6 to 10 percent, and drop back in energy content (0.7 to 0.8 mCal/lb), with less chance of over-consumption. At this stage, turnout will cause less damage to the pasture as well. Again, the most important aspect of pasture management is avoiding overgrazing – it takes a tremendous effort to get overgrazed pastures to come back, and feeding costs are exorbitant during the recuperation phase of a pasture.
A few things happen when you turn out your horses onto pasture after over-wintering (or summering in some parts) on dry hay. Besides the fact that horses will quibble, exercise a bit more, and get lots of mosquito bites (all of which reduce grazing time), the increased water content of pasture will soften the manure, reducing the likelihood of an impaction. The increased water content of ingested material acts as a buffer against any fluid losses that may occur due to increased activity and environmental temperature, at least until the grasses start to dry.
Grazing, however, can be a huge distraction, and a horse may not choose to drink enough water if finding water requires a trek. Fresh water should be nearby, and preferably not obtained from a river or pond, and definitely not from stagnant pools of water. These are unpalatable sources of water, which pose a danger for consumption of bacteria or toxins causing botulinum. Stagnant water in pastures often contains contaminated run-off from agriculture or sewage.
As grasses mature, their trace mineral and salt content generally drops to about half of their original values, but this depends of the soil and environmental conditions. There is a slight increase in calcium to phosphorous ratio as grass matures, but without consequence. Minerals and salt will need to be supplemented using salt/mineral blocks, because these quantities are not dependable. Horses spending time at pasture may also avoid salt intake, so this should be supplemented in the form of loose salt, added to the grain (2 to 3 tablespoons in each of 2 grain feedings per day) in horses that refuse salt blocks. This will protect the horse from salt losses incurred from sweating, and encourage them to seek water when outside.
Some vitamins (A and E) become sparse at the end of winter due to decay during storage. To a large extent, horses will reverse this dietary deficiency almost immediately with grazing, so supplementation with A and E can be curtailed. Exceptions are those areas with poor pasture or low geologic levels of certain minerals such as selenium, zinc, or copper. The only way to know for sure is to get clippings of your pasture analyzed for all the above constituents, but this is probably only appropriate when grazing broodmares, foals, yearlings, or competition horses.
Pastures naturally contain weeds, toxic plants, and present seeds (e.g. acorns) and leaves (e.g. red maple) that dropped in the fall, for consumption. Examples of toxic plants include St John's wart, Buckwheat, Tansy ragwort, Senecio flower, Hound's tongue, Rattlebox, Horsetail, Sage, Locoweed, Knapweed, Yellow star thistle, White Snakeroot, Fiddleneck, and Braken fern. Generally horses avoid them unless grazing fringes or just darn hungry or bored. A good exercise is to consult a botany guide and identify and remove these plants if possible from the pasture.
How Much Pasture?
You need to pay close attention to your horse's body condition, using a scoring system if possible. Pasture may be insufficient for growing horses or those in training, and horses may even get ribby when turned out on pasture. Alfalfa pasture is different from fescue or orchard grass. The results also depend on how much pasture a horse can consume, inclement weather, intensity of social interactions and exercise, predators, pests, and the quality of his teeth. On that line, pasture contains more silica (relative to hay and grain) so horses that are pastured abrade their teeth to a greater extent than horses fed indoors or loaded on grain. (Grain-fed horses require more frequent dental work.) Use body condition and performance as a good guide, but provide minerals, salt, and fresh water in every case.