There are a range of reasons why your horse might be underweight on pasture. One is that you don't have a lot of pasture. They may have some physical condition that prevents them from flourishing, or your pasture may be deficient in what your horse needs to prosper.
The first thing I would like to say though, is that it is natural - within parameters, for horses to fluctuate in weight throughout the year. We often see horses lose some weight at the end of winter, when the grasses are dormant and dried out, and the weather is colder. I don't mind horses losing a little weight in this time because they generally pick up or even get overweight when the spring grasses take off. They often look a bit scrappy at this time also, because they are about to change coats. Being a bit light at the end of winter is not necessarily a reason to panic.
How many horses can you have on pasture?
The rule of thumb that many sources will tell you is 2 acres for the first horse, and then an extra acre for each additional horse. My grandfather was a dairy farmer, and his opinion was you could run 1 horse on 4 acres, or 4 cows (as justification for prioritising the latter). Both of these are wrong, or at least misleading. Conditions make a huge difference. We use 40 acres of pasture to run our horses. We have 7 of our own horses, and at any time between 4 and 10 agistment horses. During 2018, in a prolonged drought, we entirely hand-fed all those horses. Compare that to this year, our paddocks are knee-deep in pasture at the end of winter, and the horses receive only supplementary feed. In comparison, we have many clients who have less than an acre per horse, including healthy horses and ponies that are kept in backyards.
The quality of the pasture is also going to make a huge difference to how many horses that you can run on a piece of land. Studies show that horses will increase volume when digestibility lowers, assuming that forage is available.
In summary, soil, climate, terrain, rainfall or irrigation, species of grasses planted in the paddock etc - will all influence how many horses you can run.
Instead, we can look at the individual horse. The Nutritional Requirements of Horses currently recommends horses eat 1.5% to 2% of bodyweight in forage per day. You can do the calculation for your own horse/pony. If they are not getting that in pasture, then you will need to supplementary feed hay to make up the shortfall. If your horse is underweight on pasture it may be that you just need to increase the volume of forage. This can be done simply by adding lucerne hay or grassy lucerne hay.
Geriatric horses frequently need supplementary feeding to maintain a healthy weight.
What physical conditions could prevent my horse from maintaining weight on adequate forage intake?
If your horse is receiving that 1.5 – 2% forage per day, but still not maintaining weight, there are a range of factors that you could consider:
The first is physical activity. If your horse is working hard, you will probably need to supply some extra calories to maintain bodyweight. There is a list below of excellent options.
Teeth. If your horse has an uneven grinding surface, this can make it difficult to process forage. Often horses who have bad teeth will have long pieces of grass and hay in their manures (over 2cm), so that’s one way of checking that out. Hire a dentist, especially if you haven’t done that for a while.
Parasites. Many horses look poor when they have a heavy parasite burden. Different wormers can be used for different types of parasites. You can have your veterinarian float your manures to see which eggs are present and then choose the anthelmintic that is appropriate. Failing that, Equest or Panacur are good ones to start with as they both treat adult strongyles, whereas other do not. Double-dosing ivermectin 2 weeks apart has also been a very effective regime in horses we have seen for neck threadworm. See the previous post on wormers.
Other parasites include mites, midges, ticks or lice, which can all make horses pretty miserable.
Ulcers. Please see the previous blog post on treating ulcers with products you may already have in your pantry.
Stress. There are a couple of different kinds of stress that may be a factor here. Horses that are distressed, pacing, weaving, cribbing, or windsucking often have trouble maintaining their weight just on pasture.
Another type of stress is physical discomfort. I have seen several horses that, for various reasons are unable to use their stay-apparatus. This can cause intense distress (even though they may look calm, or even dopey) because they are unable to rest. It is definitely worth consulting a bodyworker if your horse seems to have some ongoing physical pain.
Age. As horses get older they can become harder ‘doers’. They will often need supplementary feeding to maintain their weight, particularly during winter. We have two very aged mares. We have found it worthwhile to rug our geriatric horses quite heavily in winter to prevent them losing too much condition, which is then difficult to put back on.
What can you feed your horses to gain weight?
Just like you would not head straight for McDonalds or KFC if you were trying to encourage a person to gain weight in a healthy way, not all feeds are optimal. It is possible for underweight horses to founder, since that has to do with sugar content and metabolism, not weight. Some of the better options for supplementary feeding to gain weight are also low in sugar.
I have frequently seen produce stores make recommendations about feeds that will indeed cause a horse to gain weight quickly, but will then present as new problems down the track (gastric ulcers, founder, heat or more noticeable behavioural problems during oestrus, for example). While I am sure their intentions are worthy, their recommendations are usually from the reps who sell those products, and understandably, they have a bias towards those feeds with a higher profit margin.
Aside from lucerne hay or grassy lucerne, mentioned above, other good options are:
Copra meal is high in fibre, low in sugar and high in fat. It can be made into a mash for horses with poor teeth, and is generally highly palatable.
Beet pulp is also high in caloric density, makes a mash and is generally palatable. It also has the benefit of being high in calcium.
Soy hulls are low in sugar and provide an excellent source of additional fibre.
Wheat bran is not suitable for horses that have a tendency to founder, but it is an excellent source of phosphorous, and is highly palatable for fussy eaters or geriatric horses.
Rice bran is also highly palatable for fussy eaters and help to gain weight but should be used in moderation.
Quality of the pasture
You might also want to consider that sometimes, while your horse is eating enough calories, they don’t seem to be gaining muscle. This is often visible as a poor topline. In those cases often inadequate protein in your pasture can be the problem. Soy bean meal (protein pack) is a very efficient way to add extra protein. Be careful as it can be heating. Straight lucerne is also relatively high in protein, available year round, and generally well received by horses.
If you are using an off the shelf topline supplement look closely at the ingredients. These are notoriously expensive. We may be able to source these ingredients for you much more cost effectively.
Finally, this is not an exhaustive list. If you have ruled out all of the above, and your horse continues to lose weight, it may be time to consult a veterinarian for bloodwork to see if some other medical condition is causing your horse to have trouble gaining weight.
These are not the only feed options. Every horse is different and we are happy to talk to you about the feeds you choose and how to make sure your horse is getting what they need to prosper.