FEEDING BROODMARES

Mares are classified as maiden (never bred), as barren (never pregnant), pregnant or in foal, and as lactating. Their nutritional needs can be met using the two feeding programs summarized in Table 13-1. Feeding of maiden and barren mares is the same as for other nonpregnant, nonlactating horses. The other feeding program needed is for mares during their last 3 months of pregnancy and throughout lactation, during which time the amount of feed needed increases and then should be decreased in preparation for weaning.

Embryonic development and fetal growth require no additional nutrients through the first 8 months of pregnancy. Nutritional needs are the same as for the nonpregnant nonlactating horse. But fetal growth greatly accelerates during the last 3 months of pregnancy, increasing the mare’s nutrient requirements. Her requirements are further increased during milk production. The mare’s dietary energy needs increase progressively 10 to 20% during the last trimester of gestation and 80% during lactation (Appendix Table 4). As a result, mares will eat more if the feed is available. If more feed or energy is not consumed, this may decrease her milk production and reproductive efficiency as described later in this chapter. However, simply feeding more may not be adequate. As depicted in Figure 13-1, the amount of additional protein, calcium, and phosphorus needed for fetal development and milk production is greater than the amount of additional energy needed. As a result, the percentage of these nutrients needed in the diet increases to the amounts shown in Table 13-2. Therefore, simply increasing the amount fed of a diet just adequate for an earlier period (e.g., early gestation) will not provide the additional nutrients needed, resulting in the effects described in the following sections of this chapter.

Fig. 13-1. Broodmares’ nutritional needs.

For the different feeding programs, and to feed and manage optimally and economically, broodmares should be put into the following groups:

TABLE 13-2 Broodmares’ Major Nutrient Needs in Diet Dry Matter versus That in Feeds

Additional groups may be formed if warranted. For small herds, individual rather than group feeding may be more practical. Separating mares into these groups, or individual feeding, is beneficial for the safety and nutrition of the foals, and so that special consideration can be given to the needs of open and pregnant mares, those that need to gain or maintain weight, and the nutritional differences shown in Table 13-2 for each of these classes of horses. In addition, as with all good feeding programs, the amount fed should be varied as needed. The mare’s body weight and condition are the best indicators of the amount of feed needed. The mare should be moderately fleshy but not fat at foaling and fed as much as needed to maintain this weight at least until weaning. The amount of feed needed to accomplish these goals is higher with harsher climates, more work or exercise, and more milk produced, as indicated by how fast the foal is growing.

For mares to maintain body condition during pregnancy, their weight must increase by an amount equal to the foal’s birth weight plus the weight of the placenta and fluids. This is about 9–12% of the mare’s weight, with two-thirds of it gained in the last trimester. Thus, the 1100-1b (500-kg) mare should gain a total of 100 to 130 lbs (45 to 60 kg) during pregnancy, with 0.75 to 1 lb. (0.35 to 0.45 kg) gained daily during the last 90 days of pregnancy. If the mare’s weight increases more or less than this, her body condition must increase or decrease accordingly.

If the mare is provided sufficient feed, she will consume enough to attain the desired body condition, i.e., to store body fat during pregnancy for utilization late in pregnancy and during lactation. When pregnant mares are allowed to self-regulate energy intake with good-quality forage sufficient in dietary energy and protein content to meet the mares’ requirements (Table 13-2), they will consume enough to store body fat for later utilization when energy needs increase. No grain, protein, or mineral supplements need to be fed if forage quantity and quality are adequate.

If grain is fed, care must be taken not to get the mare too fat, i.e., to a body condition score of greater than 8 (Table 1-4). Although severe obesity doesn’t appear to affect the pregnant mare, the duration of pregnancy, placental weight or passage, foaling ease, or reproductive efficiency, it may decrease the mare’s milk production and as a result her foal’s growth rate. Obesity’s lack of effect on foaling ease in mares is in contrast to what is frequently postulated and stated. Obesity does lead to labor complications and dystocia in women and heifers, but not in horses or mature cows. However, the broodmare should not be kept above moderate or moderately fleshy body condition (Table 1-4) year round, as doing so may be detrimental, as discussed in Chapter 1 (see section on “Dietary Energy Excess”). If the mare is excessively overweight (i.e., has a body condition score greater than 6 to 7, Table 1-4), a weight reduction program as described in that section should be instituted. This should be done during the period from 2 weeks before weaning until the last trimester of pregnancy, but not prior to breeding, nor during the first trimester of pregnancy, nor early in lactation. Weight loss during these periods may decrease reproductive efficiency and milk production.

In addition to obesity and weight loss, excess thinness at foaling may decrease the mare’s milk production. But if the mare that is thin at foaling is allowed sufficient feed to gain weight following foaling, she will increase her milk production to a level similar to that of heavier mares at 1 month of lactation. Lactation alone requires nearly a doubling in energy intake. This, added to the amount needed for weight gain, greatly increases the amount the mare must consume. However, shortly after foaling, the mare and foal are often taken to the stud or breeding farm. Along with lactation and mothering, the mare now is in new, strange surroundings with feed and water different from that to which she is accustomed and which she, at least initially, may not consume well. If she doesn’t consume, or isn’t fed, a sufficient amount, she not only won’t gain weight, she may lose weight. In either case, the thin mare’s ability to come into heat, ovulate, settle, and maintain pregnancy is decreased. In addition, her colostrum and milk production, and as a result, her foal’s immunity and growth rate, are likely to decrease. The birth weight of the foal, however, appears to be little affected by its dam’s weight gain or loss during pregnancy, or her body weight at foaling, whether she is obese or thin.

Feeding Nonlactating Mares Not in Their Last Trimester of Pregnancy

The barren mare and the nonlactating mare during the first 8 months of pregnancy should be fed the same as the mature idle and working horse, as described in Chapter 10.

Feeding Mares During the Last Trimester of Pregnancy and Lactation

As shown in Table 13-2, if an average or better-quality legume, or grass cut or grazed at an early stage of maturity, is being consumed, this forage will generally provide sufficient dietary energy and protein, as well as other nutrients, to meet the mare’s needs during both the last 3 months of pregnancy and throughout lactation (Fig. 13-2) No grain or protein supplement is needed, provided that there are ample amounts of these high-quality forages for the mare to eat all she wants.

During the last 3 months of pregnancy and lactation, however, all forages generally are lower in phosphorus content than that needed, and grass forages generally do not provide sufficient calcium to meet the mare’s needs (Table 13-2). To prevent these deficiencies, allow the mare free access to a salt-calcium-phosphorus mineral mix as the only salt available, as described in Table 13-1. From 2 to 8 oz (60 to 240 g) per mare may be needed daily, but with a mineral mix containing 10% or more of both calcium and phosphorus, 3 to 4 oz (90 to 120 g) is generally sufficient. If grain is being fed, this amount of the mineral mix may be added to the grain. If the grain does not contain molasses, it will generally be necessary to dampen the grain when the mineral mix is added to prevent the minerals from sifting out. If mature grass pasture or hay is being consumed, in addition to more calcium and phosphorus, more protein and dietary energy are generally needed by the mare during the last 3 months of pregnancy, and particularly during lactation, than these feeds provide.

Fig. 13-2. Mares and foals on pasture. A lot of exercise and play is important for good bone and muscle development by the growing horse. Lack of exercise predisposes the foal to contracted flexor tendons. In addition, good, green growing pasture containing ample quantities of forage, along with a salt-mineral mix containing 8 to 16% of both calcium and phosphorus (Appendix Table 7), available for free-choice consumption as the only salt, will meet all of the lactating mare’s nutritional needs.

To determine if the forage being consumed meets the mare’s needs, it may be analyzed as described in Chapter 6. If the forage contains adequate energy but not protein, about 2 lbs (1 kg) of a 25 to 30% protein supplement will provide the additional protein needed. If the forage doesn’t contain adequate protein or energy density to meet the mare’s requirements, a grain mix similar to that shown in Tables 4-9 and Tables 4-10 is needed; or it can be formulated as described in Chapter 6. Along with all the grass forage she will consume, 0.5 to 1 lb/100 lbs body wt/day (0.5 to 1 kg/100 kg/day) of the grain mix should be fed as needed to have each mare moderately fleshy at foaling and to avoid weight loss during lactations, or if she is at less than moderate condition at foaling, so that during lactation she gains sufficiently to reach this condition (Table 1-4). Fat or oil may be added to the diet as described in Chapter 4 (Table 4-8) if desired. Adding fat to the diet is particularly beneficial if increased dietary energy for weight gain is needed.

After 3 months of lactation, if grain is being fed, decrease the amount by one-half, and 1 to 2 weeks prior to weaning stop feeding grain. For at least 2 weeks following weaning don’t feed any grain, and if possible, decrease the forage fed to 1.5 to 2 lbs/100 lbs (1.5 to 2 kg/100 kg) body weight daily. This helps decrease the mare’s milk production, which is beneficial in getting the foal to eat more solid feed and in decreasing excessive mammary gland distention and. as a result, the mare’s discomfort following weaning. Other procedures recommended for weaning are described in that section in Chapter 15.

As described in the following section, if the mare’s nutritional needs during pregnancy and lactation are not met, she may lose body weight and condition, and both her reproductive efficiency and milk production may be reduced. A progressive depression and elevation in blood lipids may occur, primarily in pony breed mares, when their dietary energy needs are not met.

NUTRITION EFFECTS ON MARE REPRODUCTION

Dietary Protein and Energy Effects on Reproduction

Inadequate dietary energy or protein intake may prevent ovulation, or, if ovulation and fertilization occur, may result in early embryonic death. A failure to ovulate is the major effect of a protein deficiency in mares. Mares just above moderately fleshy condition (Table 1-4) have also been shown to have greater follicular activity than thinner mares, and feeding them a low-energy diet reduced the number that ovulated. Weight loss early in pregnancy also decreases embryonic vesicle size and increases early embryonic loss. Even without weight loss, thin mares have longer intervals between foaling and ovulation, require more cycles/conception, have lower conception rates, have increased early embryonic death, and have lower plasma luteinizing hormone concentrations during diestrus than do mares that are at or above moderate body condition.

Regardless of the mare’s body condition, weight loss seems to decrease reproductive efficiency. This is quite likely the reason that, if a mare or filly isn’t bred shortly after she leaves training, or the show or performance circuit, it may take quite a long time before she does become pregnant. Following training or the show circuit, most horses lose condition while they adapt to their new environment. This is generally accentuated by their being changed from a stable to a paddock with other horses. If the other horses have been together, there is already an established order of dominance within the herd; as a result, a new mare, especially one without herd experience, may be injured when suddenly placed with the group. To prevent injury and loss of condition, and as a result a decrease in reproductive efficiency, the mare or filly leaving training or the show circuit should be kept stabled and fed individually so she maintains her body weight until in foal. She should be gradually reduced from a high-grain to a primarily forage diet. Before put with a herd, it may be helpful to put her with just one other horse for several days to allow the two to become “buddies.” Leaving her and the horses she is put with unshod, and providing ample room to circle, run away, and not get in an area where they can’t get away, decreases the risk of injuries.

In summary, the major effects of dietary protein and energy intake on the mare’s reproductive ability are the following:

Vitamin and Mineral Effects on Reproduction

The occurrence, effects, diagnosis, treatment, and prevention of a deficiency or excess of each vitamin and mineral are described in Chapters 2 and 3. Vitamins and minerals that affect reproduction, the fetus, and the neonate are mentioned here as a reminder. To ensure adequate but not excessive intake, the broodmare’s diet should contain the amounts of each vitamin and mineral indicated in Appendix Tables 2 and 3. If there is any doubt as to the adequacy of vitamins in the diet, or to ensure adequate vitamin intake, a balanced vitamin supplement such as that given in Table 3-5 should be given. In addition, tracemineralized salt, which in selenium-deficient areas (Fig. 2-4) should contain selenium, should be fed free-choice as the only salt available, or added to the grain mix as 0.5% of the total diet.

A vitamin E and/or selenium deficiency occurs most commonly in foals at birth to one month of age, and as a result of inadequate selenium intake by the dam during pregnancy or inadequate selenium and/or vitamin E intake during lactation. Although vitamin E levels are low in mature forage, pasture or hay, and a vitamin E deficiency impairs reproduction in many species of animals, this effect has never been reported in horses and there is no evidence to indicate that giving vitamin E, even in large amounts, to horses on low vitamin E diets helps resolve reproductive problems in horses or in any way increases their reproductive efficiency or libido.

An iodine deficiency, and more commonly a toxicosis, occurs in horses, or is recognized in horses, most often in newborn foals as a result of either inadequate or excessive iodine intake by their dams. Affected foals may be born dead or alive but weak, and may have an enlarged thyroid gland or goiter (Fig. 2-6) and angular leg deformities. Al-though a manganese deficiency, like one of vitamin A, greatly impairs reproductive efficiency in many species, this has never been reported or known to occur in horses. A prolonged and severe phosphorus deficiency is also thought to decrease reproductive efficiency in other herbivores but not in horses.

Although a clinically occurring vitamin A deficiency is not known to impair horses’ reproductive ability, it will decrease the amount of vitamin A secreted into the colostrum and therefore available to correct the deficiency the foal is normally born with because of minimal transplacental vitamin A transfer. If a vitamin A deficiency persists, it predisposes the foal to increased risk of infectious diseases, particularly respiratory and diarrheal diseases. In contrast to vitamin A, inadequate amounts of its precursor, beta-carotene, does impair the mare’s reproductive ability. Injecting mares not grazing green grass with beta-carotene has been reported to increase their reproductive efficiency. Because of this, beta-carotene injections, as described in the section on “Beta-Carotene Effect on Mare Reproduction” in Chapter 3, and adding 50 IU of vitamin E daily to the diet for 6 weeks prior to breeding, and throughout pregnancy, has been recommended for mares not grazing green grass, particularly for the mare that has had previous reproductive problems.

NUTRITION EFFECTS ON MARE’S MILK PRODUCTION AND COMPOSITION

Mares fed a sufficient amount of a nutritionally adequate diet produce amounts of milk equivalent to 3 to 4% of their body weight/day for the first 2 months of lactation, slowly decreasing to 2% by 5 months. The amount of milk produced varies little with duration of lactation during the first 45 to 60 days, but its nutrient content decreases steadily throughout lactation.

The effect of nutritional deficiencies on the amount and composition of the mare’s milk depends on her body condition. Mares with adequate body reserves will draw upon those reserves and not decrease the amount of milk produced, even when fed only 70 to 80% of the amount of feed needed. However, once body reserves are sufficiently depleted, milk production, but not composition, decreases. Mares with either inadequate energy or protein intake for a sufficient period of time, or without sufficient body reserves—i.e., with a body condition score of less than 4 (Table 1-4) —have reduced milk production; as a result, their foals’ growth rate is decreased.

Excess dietary energy intake, like inadequate energy in-take for mares at moderate or heavier body condition, appears to have little effect on the amount of milk produced; its major effect is on the mare’s body condition and weight. But excess, like inadequate, energy intake if sufficient for a prolonged period of time will decrease the amount of milk produced. Obese mares, with body condition scores near 9 or above, appear to have reduced milk production, as their foals grow more slowly than those of nursing mares with body condition scores of 5 to 7 (Table 1-4). Excess energy intake, in contrast to what has been speculated, doesn’t increase milk fat, energy, or protein content, but instead decreases them slightly.

Nutrient intake, above or below that needed, appears to have minimal effect on milk composition. Wide variations in the mare’s calcium and phosphorus intakes have been shown in some studies to have no effect on their concentrations in the milk. In other studies, although dietary phosphorus had no effect on milk phosphorus content, calcium concentration was 40% lower in milk from mares receiving one-third the amount of dietary calcium needed, but milk calcium concentration was not different in milk from mares receiving up to 2.5 times more calcium than needed, than it was in those receiving just adequate dietary calcium. There appears to be no correlation between the amount of inorganic potassium, zinc, copper, magnesium, and iron ingested and the amount in the milk. In contrast, milk iodine and possibly selenium concentrations directly correlate with the amount of each of these minerals consumed, as described in Chapter 2.

In summary, variations in the amount of most nutrients ingested by the mare do not affect their concentrations in her milk. The major effect of variations in energy intake is on the mare’s body condition, not the amount of milk produced or its composition. The objective of feeding programs for the lactating mare, therefore, is to keep her at the desired body condition, rather than to try to influence her milk production or composition.

NUTRITION PROBLEMS IN BROODMARES

A summary of nutritionally related problems that occur primarily in broodmares is given in Table 13-3. Poisonous plants and feed-related poisonings as discussed in Chapters 18 and 19, respectively, may affect all horses, and not just broodmares. However, some, such as fescue toxicosis, occur only in broodmares and growing horses. Care should be taken that fescue hay or pasture-forage is not consumed during the last trimester of pregnancy unless it is known to be endophyte free. Infested fescue consumed during this period may cause abortion, prolonged gestation, a thickened placenta resulting in difficult foaling, a decrease or absence of milk production, and the birth of weak or dead foals. The mare also should not consume sorghum forage, sudan grass, Johnson grass, Sudex, or other sorghum-sudan grass hybrid forages. All produce a substance that may be converted to cyanide (prussic or hydrocyanic acids). As described in Chapter 18, sufficient cyanide causes incoordination and bladder paresis in all horses, and may cause pregnant mares to abort or give birth to foals with fused joints. A number of additional plants that may cause the production of foals with physical defects (i.e., are teratogenic) are listed in Table 18-11.

There has been concern that transport of mares early in pregnancy may increase the risk of abortion, but studies have refuted this. However, prolonged transport, exertion, or stress from any cause, particularly during late pregnancy and lactation, may result in a sudden decrease in the horse’s blood calcium and magnesium concentrations. This condition is described in the section on tying-up or hypocalcemic tetany in Chapter 11. The likelihood of this occurrence is greatly increased by a decrease in, or absence of, feed intake during this period.

A progressive depression associated with an increase in blood lipids (hyperlipemia) may also occur during lactation or late pregnancy, when energy needs are high but intake for any reason is low. This may occur as a result of inadequate feed quality or availability, or secondary to inadequate feed intake as a result of inappetence or inability to consume food. Inadequate feed intake is most often due to the horse’s being sick or having esophageal or gastrointestinal damage or obstruction. Hyperlipemia secondary to not eating may occur in any horse from one day of age to old age, and in any body condition, but occurs most often in thin horses. However, as a primary disease, not secondary to the horse’s not eating, hyperlipemia occurs most frequently, and is a classic problem in obese ponies during late pregnancy, in lactation, and transit, but may occur in nonobese, nonlactating, nonpregnant ponies or horses.

TABLE 13-3 Nutritionally Related Problems in Broodmares^a

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