Page 63 - August 2015

Withers height is a close measure of skeletal development in young horses.
growth at 3 months of age will influence what a foal will be at 2 or 3 years of age,” he explains.
At many of the breeding farms he worked with, broodmares and foals are kept on pasture. Some mares are brought into stalls for foaling, but most foal-out in the fields. “One thing we pay close attention to, in the research we’ve been doing (and for the health of the animals), is trying to sort out the relationship between the environment and the animal. The environment includes the pasture, climate - all the things
that surround and affect the animal and its physiology. One of the things we are able to do
is look at the physiologic mechanisms that are a basis for the pasture/animal relationship,” he says.
Staniar has done a lot of work examining growth patterns. “The long-term growth pattern for horses has already been well defined. I think of this as their genetic potential, and there are some significant breed differences in that curve, but it’s basically a sigmoid curve. The foals grow in utero - slowly at first and then a period of exponential growth just before birth. After the foal is born, it continues to grow very fast for a while (the fastest growth is during the first few weeks after birth) but as the animal matures, the growth slows and then reaches a plateau,” he says.
“We are also trying to understand short-
term patterns of growth. Long-term patterns are genetic, whereas short-term patterns are affected by seasons, environment, etc. The latest NRC (National Research Council) update has a growth curve that characterizes the growth of horses
of many breeds. This data was mainly from Thoroughbreds, Quarter Horses, etc., and lacks
The environment in which you are weaning a foal is also important; a foal may have a negative impact on growth if weaned in the hot summer with poor pasture, whereas a foal will likely continue a strong growth pattern if weaned in early fall when pasture is re-growing.
some of the pony and draft breeds,” says Staniar. But, there are some periods during growth
where this doesn’t fit due to short-term growth patterns. These are separate and apart from the genetic patterns and involve the environmental influence (nurture versus nature). “How does temperature, available feed, weaning, etc., affect patterns of growth for instance? These environmental influences are probably more important to the horse owner raising foals - in the day-to-day care and management of the foal - than the long-term growth patterns,” he says.
There are many questions. “One is how date of birth affects the foal. If it’s born in January versus April, does this influence growth?
We’ve found that the short-term pattern is very different for a foal born in January versus a foal born in April. Conditions in which they are growing up are different, at different ages. The tricky thing is the intersection between age of the animal and its potential for growth, and the environment at that particular time (feed available, temperature, and so on),” he explains.
“When you compare the growth of foals born in January to those born in April, the April
foals are growing more rapidly at a young age because they are born into a more nutrient-rich environment with green grass. Foals born earlier will be heavier at that time of year because
they are 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.
When discussing growth in foals, we need to differentiate between weight and height. “Weight is what we talk about in cattle or other livestock species. Weight is not so important in horses. It plays a role and is one of the factors we look at, but withers height is a closer measure
of skeletal development in young horses,” says Staniar. Many breeding farms measure withers height, but you won’t find a growth curve for withers height in the NRC recommendations. There’s not much research that’s looked at growth patterns in terms of withers height, even though it’s what we most often measure.
“There are still questions we have no answers for. We want a smooth growth curve in a young horse. We think this is best because we don’t
Long-term growth patterns are influenced by genetics, while short-term grown patterns are affected by seasons, environment, available feed, weaning, etc. - it’s nurture versus nature
SPEEDHORSE, August 2015 61
equine health