Over an 11-month gestation period, the mare faces varying nutritional needs. Overfeeding or underfeeding during the course of the reproductive cycle can impact foal development and reproductive efficiency. Commonly encountered nutrition-related problems in feeding mares are:

• Overfeeding during pregnancy.

• Underfeeding during lactation.

REPRODUCTION
Since conception rate is influenced by body condition of mares at breeding, it is extremely important that a mare be in optimum body condition at breeding time. Overfeeding can lead to obese mares and underfeeding a lactating mare can cause diminished body condition. Fat and thin mares have lower conception rates compared to mares in good body condition. Additionally, fertility can be decreased if mares are deficient in protein, phosphorus, selenium, iodine, or vitamin A.

Conception rate might be improved by increasing the concentrate intake a few weeks prior to breeding. Horses gaining weight at the time of breeding appear to have good conception rates. If a mare is overweight, a body weight reduction program should take place between weaning and prior to the last 90 days of gestation.

A mare should enter the breeding season in optimum body condition. Based on research by Henneke et al. (1983), it was suggested Quarter Horse mares should have a minimum body condition score of 5 (moderate) at breeding time.

The feeding program needs to be tailored to maintain the horse in fit, but not fat, condition. Regardless of the amount of feed consumed, horse owners still need to ensure the horse's nutrient needs for protein, minerals, and vitamins are met. Thus, when decreasing feed intake to reach a trim, thrifty body condition (not too fat, not too thin), it may be necessary to slightly increase concentration of protein, minerals, or vitamins.

The reproductive cycle of the mare can be divided into three nutritional phases:

• Non-lactating, pregnant mare prior to last 90 days of gestation.

• Pregnant mare during the last 90 days of gestation.

• Lactating mare.
       - Early lactation (first three months of lactation)
       - Late lactation (three months to weaning)

GESTATION
The health of the foal is greatly influenced by nutrition of the mare. Insufficient nutrient intake during gestation can have serious consequences on foal development and, as stated previously, reproductive efficiency. The nutrient needs of pregnant mares in early gestation are similar to a mature horse at maintenance. The exception is a pregnant mare which is nursing a foal. In this situation, the pregnant mare should be fed based on a lactating mare's nutrient requirements.

A common feeding mistake is to overfeed during gestation. A pregnant, non-lactating mare in optimum body condition prior to the last 90 days of gestation should be fed as a mature horse at maintenance. Mares in thin body condition should be fed to gain weight. The amount of grain ration needed during the non-lactating, early to mid-gestation phase depends on forage quality and body condition of the mare. Good quality hay or pasture and appropriate supplement will usually meed nutrirnt needs during early pregnancy for non-lactating mares in good body condition. A small amount of grain ration maybe needed if hay or pasture does not meet energy needs and/or when a mare is in thin body condition.

Last 90 Days of Gestation
Increasing nutrient level during the last 90 days of gestation is imperative as fetal growth rate is greatest during this time. The fact that 67% of fetal growth and 90% of fetal bone development occurs over the last three months of gestation supports the need for increasing nutrient intake to meet growing fetal needs. Meeting vitamin and mineral needs during this stage of fetal growth is essential. An example of a mineral deficiency during gestation is the development of rickets in foals born to mares deficient in either calcium or phosphorus. Another example is calcification of a foal’s aorta due to magnesium deficiency.

The non-lactating, pregnant mare’s nutritional needs do not significantly increase until the last 90 days of gestation, and even then nutritional needs of pregnancy are not overly excessive. Energy needs increase up to 20% over the last three months of pregnancy and protein needs increase 32%. To meet increasing nutrient needs during late gestation, a small quantity of grain ration is usually fed. As feed intake increases during late gestation, so does body weight. Body weight typically increases up to 15% during the last three months of gestation.

LACTATION
During lactation, the mare needs to be fed to satisfy milk production requirements and maintenance needs. Nutrient needs of the lactating mare depend on amount of milk produced. Peak milk production is reached approximately two to three months into lactation. After this peak, nutrient requirements diminish. Notable nutrient increases during late pregnancy and lactation include energy, protein, calcium, phosphorus, potassium, magnesium, and vitamins A, D, and E. Insufficient nutrient intake will suppress milk production and reproductive efficiency (ability to breed back while lactating). Mares in thin body condition may not conceive until after foals are weaned and body condition increased.

Early Lactation
Early lactation is the period of greatest nutrient demand faced by the mare. Consequently, the amount of grain ration fed needs to increase compared to late gestation feed intake. The amount of grain ration fed will depend on quality of forage, level of milk production, and mare body condition. The nutrient composition of the grain ration also influences the amount of it to feed. The higher the level of nutrients in the grain ration, the less grain ration required.

Late Lactation
During late lactation (three months to weaning) milk production decreases to 2/3 the level produced during early lactation. Consequently, nutrient needs decrease. Also, during this time, the foal starts to consume creep feed and forage, decreasing its dependence on mare’s milk. The level of grain ration fed is typically 30-40% of the total feed intake. Again, the amount of grain ration fed varies depending on previously mentioned factors.

MILK PRODUCTION
Estimates of milk production are 3% and 2% of body weight during early and late lactation, respectively. The amount of milk produced by mares varies greatly. While mare’s milk provides nutrients for the foal, it should not be relied upon to provide the sole source of nutrients for foals. The highest dry matter concentration of mare’s milk occurs very shortly after birth and drops significantly 12 hours after parturition. Energy provided by mare’s milk is inadequate to meet foal needs one month after parturition. Protein is usually insufficient to meet foal needs during the entire lactation period. Calcium, phosphorus, iron, and copper are also notably low in mare’s milk. As lactation progresses, fat, sodium, potassium, and other nutrients decline. Due to insufficient and declining nutrient levels in mare’s milk, a good creep feeding program may help keep foal growth and development at desired levels. A word of caution: Young, growing horses should not be overfed (grow too rapidly). Overfeeding can lead to bone developmental problems.

WATER
Water is also a nutrient and a mare’s need for water should not be overlooked during any phase of reproduction. Milk production increases the need for more water as does hot weather. Limited water availability could negatively impact milk production. An unlimited supply of good-quality, clean, cool water needs to be available at all times.

FEEDING
As with any horse diet, feedstuffs (namely grain and forage) fed to horses should be good quality. Forage is the mainstay of horse diets. Since forage can vary greatly in nutrient content, it is very important to have forage analyzed for nutrient content and then, supplement as needed to ensure mare nutrient needs will be met. Grain and supplement can be fed as a grain mix according to nutrient requirements, forage quality, and body condition.

Table 1  lists nutrient requirements, Table 2 lists nutrient concentrations in the total diet, and Table 3 lists expected feed consumption guidelines.

SUMMARY
Feeding mares is not a complicated process. Horse owners need to monitor mare body condition year-round and feed appropriately based on body condition and phase of reproduction. Overfeeding during gestation and under-feeding during lactation are serious problems that can affect foal development and reproductive efficiency. A very good mare management program and high-quality feeding program are needed to enable mares to conceive and produce a foal every year.

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