After much time, effort, and cost in getting our mares bred, we are now in the short rows.  Many mares are in the last 2-3 months of gestation, the last trimester.  During this time, the fetus will experience 75-80% of their development.

The nutrient requirements during the first and second trimesters did not differ significantly than they would be if she were not pregnant.  During early gestation, we feed the mare based on her stage of production, lactation, work, etc.  In the last trimester we begin to alter her nutrient intake to help meet the demands of the developing fetus.  If we short-change in these nutrients, she will draw on her body reserves to meet these demands.  This often leaves the mare with reduced reserves, especially energy, when entering lactation.  We want our mares to be a BCS of about 6 when the foal is born.

Prenatal nutrition for the mare is as important as that for humans.  A nutrient deficiency during pregnancy can have a profound effect on the horse later in life.  While this Blog is about feeding the mare in the last trimester, that is not to say her nutrition earlier in pregnancy is any less important.  While there is scant data specifically in the horse, there are multiple studies in humans and other animal species that show the importance of fetal programming and how nutrient deficiencies could lead to health issues later in life.  This is related to both under and over feeding.  Again, that is another Blog related specifically related to Fetal Programming.

This blog is directed to the requirements for energy, protein, calcium, and phosphorus during the last trimester of gestation.  Maintenance and the first month of lactation requirements for these same nutrients are included as a reference.

As the mare enters the 9^th month of gestation her energy requirement increases about 15% over that needed for maintenance. Energy needs during the 10^th and 11^th months are about 20% and 28% higher compared to maintenance, respectively.  The DE intake needed for each of these months is 18, 20, 21 Mcals/day, respectively.  A good quality legume hay, e.g. alfalfa, fed at about 2% of the mare’s body weight (BW) will supply each of these requirements (table 1 and figure 1).

Her protein requirement increases more as a percentage of her maintenance needs as her pregnancy progresses.  Compared to maintenance, she needs 26%, 33%, and 42% more protein in months 9, 10, 11, respectively.  She requires about 1.4 pounds of protein per day at maintenance, 1.76, 1.85, and 1.97 pounds of protein in months 9, 10, and 11, respectively.  As before, feeding alfalfa at about 2% of her BW/d (table 1 and figure 2) will do a good job of supplying those requirements.

The primary mineral requirements that are changed during gestation is calcium (Ca) and phosphorus (P).  Both increase relative to maintenance and stabilize in months 9. 10, and 11 (table 1 and figure 3).

During the first month of lactation, the nutrient demands placed on the mare are as high as any time in her life.  Her energy requirement is equal to that of a horse in race training.  Her protein, calcium, and phosphorus are also significantly increased (figure 4).  Supplying sufficient feed and dietary energy during this time without causing digestive issues is a major issue.  Feeding only a forage, even as energy dense as is alfalfa will fall short of meeting her energy demands in early lactation (figure 5).  Feeding alfalfa at 2.5% of her BW during this time will still not meet her energy demands.  As shown in table 1, feeding about 20 pounds of good quality alfalfa will supply all her nutrients during the last trimester.  While her nutrient requirements are being met, I encourage horse owners to begin adding a quality grain source to her diet during this time.  I recommend this for a couple of reasons.  First, her forage intake may decrease due to the developing fetus taking up space in the abdominal cavity.  More importantly, she is going to need grain to supply her nutrient needs post-foaling.  If we have not begun to adapt her digestive system to the grain (carbohydrate/fat) and must begin feeding it rather quickly, we may cause a laminitic episode or induce a colic issue.  If the mare has a BCS below 6, you need to begin feeding grain even earlier, probably month 7 or 8.  I like to feed the same feed I am going to feed during lactation.  My preference is a feed formulated for growing horses.  If she is already on this feed, I will not have to change the feed post-foaling.  The foal will begin eating with her at about 2-3 weeks of age and I want the foal to be eating a feed designed for growing horses.

Table 1. Nutrient Requirements Maintenance, Late Gestation and Early Lactation. For a 500 kg./1100 lb. horse

Figure 2. Protein Required in Late Gestation and Early Lactation