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How feeds affect fertility in mares

What is fertility in horses

In general, mares come on season (cycle) every 21 days, and have a gestation of 340 days or 11 months. For a breeding mare, optimum fertility is a live foal every 12 months, and so there is window of opportunity (30 days) for mares to be served and become pregnant again.  Mares can be served on foal heat (7 days after foaling) and then again on 21 day cycles.
Therefore for a mare to have a foal every year, she must

Some science … relationship between glucose and ovulation, gestation and lactation

As with most female animals, ovulation in mares is a function of ‘glucose production rate’ and circulating levels of blood glucose. Mares need rising planes of nutrition, ie rising total energy intake, and rising glucose production rate to ovulate. Therefore, if  mares are in poor condition,  the glucose production rate and circulating levels are falling and mares may not ovulate.

Lactating mares also have a huge ( at least 2 fold)  added requirement for energy to produce milk. If the mare is not being supplemented with additional energy, and she is in poor body condition, she will convert body mass (fat and protein) into milk. Glucose productions rates and blood glucose levels will fall and she will not cycle.

Mares in good body condition at foaling can milk ‘the fat’ off their backs – [like dairy cows], ie they can convert body tissue into milk. This allows them to produce the quantity of milk required by the growing foal, to endeavour to maintain blood glucose levels. To avoid the mare losing too much bodyweight after foaling, it is essential that the mare be given a well balanced diet at least 4 months before and 4 months after foaling. For pregnant mares, further improvements in milk production can be made by increasing body condition prior to foaling, provided the diet is balanced.

How do I know my horse is fertile

Many factors affect the ability of a mare to produce a viable egg which can be fertilized and that will implant successfully.

These factors include

The most important non disease factor influencing fertility is body condition. This is determined by diet composition and total energy intake.  Condition is measured using a 9 point score from poor/emaciated (1) through to extremely fat/obese (9) .  Mares should be condition score 5-6 at foaling and at mating.

What influences fertility in horses

Mare fertility can also be affected through overfeeding, obesity and associated issues including increased length of anoestrus or the length of time between cycles. One study reported that obese mares exhibited a significantly longer duration of the oestrous cycle, significant increases in circulating concentrations of leptin and insulin, and decreased insulin sensitivity and concentrations of thyroxine compared with feed-restricted mares (Vick et al 2009).  It is suggested that some obese mares may suffer insulin resistance.

In summary, the accepted feeding practice can be stated as “An adequate energy intake, matched to the mare’s needs, is essential to achieve and maintain an optimum body condition for breeding.  The feeding program must therefore be tailored for maiden mares, dry mares and lactating mares.

Effects of NSC [non-structural carbohydrates] on fertility

Energy is derived mainly from the sugar and starch (NSC or non structural carbohydrate), and also from digestible fibre, oil and protein.  It is only in recent years that the negative effects of high levels of sugar and starch (NSC) content of feeds on horse health have been identified (see previous articles in this series in Horse and People). High NSC feeds (>12%) can also cause serious metabolic disorders that will affect the mares health, including tying up, laminitis, gastrointestinal ulcers and difficult temperament. It is now well recognised that horses can suffer from Insulin Resistance (IR) in the same manner that humans suffer from Type II diabetes, and that the culprit is sugar and starch intake (NSC >12%) .

Feeding high energy (high NSC) feeds will increase blood glucose levels but not necessarily increase glucose production rate because of the rapid feedback mechanism of insulin secretion to counter the glucose ‘spikes’ and the partitioning of glucose into fat cells. This causes obesity and infertility, arising mainly from irregular anoestrus, and possibly from polycystic ovarian syndrome.

The conundrum therefore is what do we feed to improve and maintain body condition, and not cause insulin resistance?

How diet helps fertility in horses

The feed must provide a high DE (digestible energy) to increase the circulating levels of glucose, maintain insulin sensitivity, and prevent obesity to optimise the physiological conditions for the mare to conceive and produce a healthy foal. The aim is to select high DE feeds that do not cause high levels and spikes in glucose, and hence insulin. These feeds should therefore be low NSC (<12%) and high oil.

The main factors to consider are DE and NSC content, and use of “slow feed”.  All horses are slow feeders, ie they eat continuously during the day.  Slow feeds that are high DE, and provide glucose continuously throughout the day, and do not cause glucose spikes are ideal and represent a more natural diet.  In the study by Richards et al, unpub, several feeds were fed to horses at pasture to determine the effect on circulating glucose and insulin (see below).  The high NSC pelleted (25%) and sweetfeeds (34%) feeds caused glucose and insulin spikes. By comparison, the meal CoolStance (11%) did not cause a glucose spike, and yet provided a high DE intake from the oil. This is an example of an ideal slow feed.  Feeding hay in hay feeders is also an example of a slow feed, however hay will not provide the DE required by the mare for optimum fertility.



NSC (%)

DE (MJ/kg DM)

Glucose (over 6 hours)

Kikuyu pasture





Coolstance coconut meal















Recommended Stance Horse Feed for fertility in horses

Feeds for mare fertility. Select feeds that have

The Stance Equine Feeding System outlines several low NSC feeds.  These include