by Heather Smith-Thomas

Foals grow very swiftly during the first months of life. Genetics, feed and environment all play a role in the foal’s growth and skeletal development. W.B. (Burt) Staniar, PhD (Assistant Professor of Equine Nutrition at Virginia Tech) is involved with research at the Middleburg Agricultural Research and Extension (MARE) Center at Middleburg, Virginia. About 50 Thoroughbred broodmares are kept at the Center and bred every year. “The research I am conducting focuses on the foals’ first 18 months of growth, and how nutrition provided by the pasture and supplements we feed influence that growth,” says Staniar.

“Our broodmares and foals are kept on pasture at all times. Some mares are brought into stalls for foaling, but most of them foal out in the fields. It is always amazing to see how quickly the foals born in the field are up and running. One of the things we pay close attention to, in the research we’re doing and for the health of the animals, is to sort out the relationship between the environment and the animal. By environment I mean the pasture, climate and all the things that surround and affect the animal and its physiology. One of the things we are able to do here is to look at the physiologic mechanisms that are a basis for the pasture/animal relationship,” he says.

From a growth rate standpoint, foals at the MARE center are handled differently than those on most Thoroughbred breeding farms. “Our breeding season typically runs from mid-April to mid-June, a bit later than that of the Thoroughbred industry. This means that over the last 12 to 15 years, 95 percent of our foals have been born in April and May. We’ve found it has great benefits to the health of the animals. Instead of having them born at a time our pastures are at their worst (in February) and temperatures are very low (which can stress the young foal), they are born when pastures are optimum,” says Staniar.

The mare in early lactation has the greatest nutrient and energy demands. “We feel there is real benefit in synchronizing what the animal is doing with the environment. This is the way the horse would have done it in the wild,” he says.

Mares with young foals are on pasture when forage is best. “Our pastures reach a peak about the end of April and beginning of May. The mares in early lactation are eating some of the best forage available. The temperatures are good for the foal. Foals begin to nibble grass at that age, and get some benefit from the good pasture,” he says.

“We have no need to keep mares under lights because they are naturally cycling the time of year we are breeding, so our breeding rates are very good. We often accept donations of broodmares that have a difficult time breeding, and find that breeding in May and June is the best solution for them,” says Staniar.

“When you compare the growth of our foals to others in the Thoroughbred industry, our foals are growing more rapidly early on (at a young age) because they are born into a better environment. Foals that are born earlier are obviously going to be heavier at that time of year because they are a month or two older, but they didn’t grow as fast at that young age.” The researchers study the relationship between the pasture and the animal, looking at growth rate, to see how they can match up where the animal is and its ability to grow.

The foal’s growth rate slows as it gets older. Foals grow most rapidly during the first weeks after birth. “People typically think of growth curves, with an x axis depicting age (in months, day of year, or various time increments) and a y axis with weight of the foal. A more descriptive way of looking at that is to put on the y axis the average daily gain. We might use killigrams per day, for instance. This will be a decreasing curve, since foals are growing at the greatest rate early on--and it steadily decreases,” he says.

One of the issues with growth is trying to avoid various developmental problems that can occur. From a research standpoint, and also from a management standpoint, we need a definition of growth. What kind of growth are we looking for? Do we want rapid growth? Maximum growth? Slow growth? What kind of growth will meet our objectives? “I think our objectives are to maximize the opportunity for that animal to realize the athletic potential that’s a part of its genetics,” says Staniar.

“Optimum growth is what we are looking for, but defining that is difficult because it may be different for each individual--and the end result is often 2 to 5 years down the road. It’s hard to determine how the growth at 3 months of age will influence what a foal will be at 3 years of age,” he explains.

“When we wean these foals, we also look at growth rate because weaning usually has an influence on it. We wean our foals at 6 months at the MARE Center. For us, this is typically in October. Conveniently, October is the second month of the year when our pastures are at their highest quality. We have a peak in pasture nutrient and energy content in April and again in October. That’s convenient for us because when we’re stressing those foals by taking them away from the mare, and changing the diet from milk to forage, they are being weaned onto really good pasture,” he says. By 6 months of age a considerable portion of the foals’ diet is already being provided by pasture.

“This is very different from a management situation in which foals are born in February and weaned in July. Our two low points for pasture are February, and July-August, in Middleburg. It will be different in different areas, but in our management practices for growing foals, we try to take advantage of the forage that is available--when it’s available.” It helps to work with nature.

“We still see a decrease in growth during weaning, because the animals are stressed somewhat, and though conditions for forage in October are at a high, from that point on they are decreasing as we move into winter,” he says. There’s a change in environmental conditions and the body starts to conserve energy for thermal regulation (to keep warm) and for building a winter hair coat. There’s not as much energy put toward growth. It’s getting colder, pastures are declining in quality, and all the signals to the animal are that the environment is changing and it needs to get ready for winter.

“In all the growth data I’ve looked at, there’s a decrease in growth rates in the winter months, with February usually being the lowest for us here,” says Staniar. “The body is conserving energy for other purposes and decreasing the growth rate, and this happens in all species. But the decrease at this time represents an opportunity for us as managers. If you realize this is going to occur, you can be providing the animal with more energy, and an environment that doesn’t allow the growth to decrease quite so much,” he says. You can counterbalance some of the negative effects of that first winter for the weanling.

“You can try to make up for some of the deficiencies in the environment. For example, it’s an opportunity to provide a better quality hay so there is more energy available to the horses. The more an animal decreases growth at this point and conserves energy (putting it toward maintenance instead of growth), when spring comes and there’s good grass again, the more the animal tries to catch up,” he says. This growth spurt is called compensatory growth and it happens in all species.

“The more the young animal decreased its growth through that first winter, the more likely it will have an even greater increase in growth the next spring,” he explains. And it is this type of growth spurt that has been indirectly linked with developmental problems in the growing skeleton.

“Our animals are out on pasture all the time so we see this natural pattern of growth occur,” he says. If you can compensate for the decrease in growth during winter and be thinking about it in spring (not complicating a growth spurt by feeding high energy feeds at that time), this can smooth out the peaks and valleys of the young horse’s growth and potentially help minimize some of the risk for various types of DOD (developmental orthopedic disease).

“Here in Virginia, March is a month of change for us. The first of March is a low point and by the end of March the pastures are beginning to improve and the animals are beginning to put energy toward growth. If you look at their growth curves, you see a big change in average daily gain going from February to April. So we try to compensate as much as possible. It’s not the time of year to be giving these foals even more energy in their diet. While it has not been proven scientifically (the research is spotty), there is a perceived connection between rapid growth and developmental problems,” says Staniar.

“So this is something we want to minimize. The mistake some people make is to take the animal off all energy altogether to try to halt the rapid growth. It’s more important to look at the big picture, and the growth of that animal and what your objectives are over the first 2 years--and how you can try to moderate some of those short term changes,” he says.

For example, if you have a foal whose sire and dam are both large animals, your expectation is for that horse to be large when it grows up, with a strong growth rate. Thus you don’t want to hinder the growth rate; you just want to maintain what its genetic potential and long term growth curve should be, with as few deviations as possible. You try to minimize those deviations, and smooth out the fits and starts, slows and spurts; you need to plan ahead for proper feed and management.

“One of the things we work on (an idea that’s still in development) is a pro-active or dynamic management tactic, understanding the components of the environment and how they change in relation to the animal and its growth. We want to be able to compensate before problems occur, before the changes in growth occur. Before the animal begins to decrease in weight, you need to alter the feed/management,” explains Staniar.

In foals that are on pasture, you often don’t notice when the pasture begins to decline. “By the time you notice changes in the pasture or the animal, both have changed. The rough measurements we use, like regularly weighing the animal, are not enough. By the time we see a change in rate of gain, much has already happened at the cell level and it would be better to alter the management/feed before that change in weight occurs,” he says.

“For people who are weighing their foals periodically, these changes must be made before the animal begins to decrease or increase in weight gain, or you are already after the fact. I’m not saying it isn’t important to be weighing your animals (and this is something we do with ours), but by the time you see changes in the weight of those animals, a lot of changes have already occurred at a molecular/cellular level in the cartilage and bone, and you are behind the eight ball,” he says. To try to correct at that point, you may be overcorrecting.

“Thus it’s important to have a pro-active, dynamic view and understand the animal in its relation to the environment and be able to manage accordingly.” You must try to stay in step with what’s happening as the foal grows.

“A lot of the research I do here is trying to understand how nutrition and energy in the diet are influencing the cartilage and bone development. When studying the growth of foals through their first 18 months of life, I’m not as much worried about muscle development or behavior as I am their skeletal development. In my mind, the first 18 to 24 months is when that skeletal framework is developing and maturing.” The end goal for most of us is being able to reach that animal’s potential for athletic performance, and the skeletal framework is the structure from which that athletic performance will be realized.

“So I want that skeleton to be as strong and healthy as possible, at the end of two years. I want it to be able to facilitate all the genetic capabilities that animal has, to become an athlete. The research we are doing is beginning to bear out some of our thoughts on this. For example, I have a graduate student looking at the cartilage in the young horse; we are trying to understand how different ways of feeding and different sources of energy are actually affecting gene expression in the cartilage itself,” says Staniar.

“We are looking at the science behind it all and trying to understand how the cartilage actually responds to different feeding management techniques. We are studying changes in the expression of particular genes in the cartilage of our foals in response to changing pasture conditions and dietary energy sources we provide. We know these changes occur during growth, and none of this is a surprise, but we are trying to find out why these things occur, why growth problems occur, and understand the mechanisms involved. Once we understand these, it can give us a tool for how to manage the growth, and address the various problems.”