NUTRITION LIBRARY

Pregnant mares must consume enough nutrients to both maintain their own body weight throughout the pregnancy and also to create new tissues, those of the fetus and those that support the pregnancy such as the placenta. Due to the very small amount of tissue deposition in the first months of pregnancy, providing the mare is in good weight and not working, she can be fed as a horse at maintenance for the first 4 months.  However, due to the development of the tissues supporting the pregnancy, starting in month 5 her nutrient requirements change.

Energy

Fetal growth is greatest during the last 60 days of gestation and therefore traditionally it was not felt necessary to increase energy intake until the 3^rd trimester. However, while fetal growth is greatest in the last trimester, more recent research (Reynolds et al., 1986 and Fowden et al., 2000) has shown that during the 2^nd trimester there is an increased energy requirement for development of placental tissues. Therefore, the guidelines laid out in the 2007 National Research Council’s (NRC) Nutrient Requirements of Horses recommends that energy in the diet be gradually increased starting at the 5^th month of gestation with a greater rate of increase in the last 3 months (See Table 1).

Table 1: Energy requirement of the 500kg (1100lb) mare by month of gestation.

It is expected that mares should be seen to gain weight during the last 3 months reflecting the increasing weight of the foal. Over the entire pregnancy mares can be expected to gain 12-15% of their non-pregnant weight and foal birth weight is estimated to be 9.7% of the mare’s non-pregnant weight. So a foal born to a mare whose non-pregnant weight is 1100lbs will weigh around 107lbs at birth. Mares who do not gain weight during the last trimester will be utilizing their own body energy stores to support fetal growth. This puts the mare at a disadvantage after foaling when she will need those body energy stores for milk production. Mares who do not receive the necessary energy during this time may foal late in an attempt to have their foals be at the correct weight at foaling. If the mare did not enter gestation with a body condition score (BCS) of 5 or above these energy intakes would need to be increased further in order for her to gain weight and have a BCS of 5 by the 9^th month.

The best starting source for the extra energy requirement is from hay, however as the fetus grows the amount of space within the body cavity for the digestive tract becomes proportionally less. Your broodmare may not be physically able to eat the amount of hay that would be required to meet her energy requirement. This is especially true if you are feeding a lower quality hay with a higher proportion of non-fermentable fiber. Ideally, as for all horses, hay fed to mares and foals should be analyzed for all nutrients including the hays neutral detergent fiber (NDF) and acid detergent fiber (ADF) content which are measures of hay quality. NDF estimates the amount of cell wall material in the hay at the time of cutting. The more mature the hay is at cutting the greater proportion of cell wall material, and while NDF does not appear to be the sole determinant of voluntary dry matter intake in horses some studies have shown that as the proportion of NDF increases intakes decrease. Hays with an NDF proportion greater than 65% should be avoided, especially for broodmares and youngstock, a desirable NDF is 55% or less. ADF (cellulose and lignin) increase as the plants in the hay become more mature, the higher the percentage ADF the lower the digestibility of the hay. ADF values over 45% are of little nutritive value and ideally the ADF proportion should be below 32% for broodmares and youngstock. It should be noted that maturity is not a reflection of the cutting (1^st, 2^ndcuttings etc) it is a reflection of the maturity of the plants when they are cut.

If your mare will not physically eat enough hay to meet her increasing energy needs then you will need to turn to other more energy dense feeds such as grains or commercially compounded feeds. The benefit of starting this before foaling is that the mare will have adjusted to this type of diet before reaching the very demanding metabolic state of lactation. As we saw in table 1, in the last month of gestation the 500kg mare requires 21.4 Mcal per day this increases to 31.7 Mcal once the foal is born and the mare is in the first month of lactation!

Protein

Protein is needed to increase lean body mass so it comes as no surprise that the mare’s protein requirement increases during the last trimester. Interestingly there have been studies (van Niekerk and van Niekerk 1997a) showing that feeding diets deficient in protein may lead to a slower return to ovulation possibly due to lower progesterone concentrations post foaling. This is an important consideration if you are planning on re-breeding your mare. In early pregnancy low protein levels have been linked to lost weight and early fetal loss (van Niekerk and van Niekerk 1997b).

The crude protein requirements of the 500kg pregnant mare are shown in table 2.

Table 2: Crude Protein requirement of the 500kg (1100lb) mare by month of gestation.

If the 500kg mare (not accounting for extra weight gained during pregnancy) is fed 1.5% of her body weight per day as grass hay (just over 17.5 lbs) with a crude protein level of 10% she will be consuming 800g of protein, enough to meet her requirement through the 9^th month. If we assume that she does not eat any greater amount of grass hay in her 10^th and 11^th months then she would need some protein supplementation from some other source. A lot of people feed their broodmares some proportion of their hay intake as alfalfa because of alfalfa’s higher protein content. If the grass hay intake of our 500kg mare was reduced to just over 14 lbs and just over 3 lbs of a 18% crude protein alfalfa was fed the total crude protein intake would be 920g exceeding the 11th month requirement. As you can see feeding the common practice of feeding 30-50% of hay as alfalfa easily insures protein needs are met.

However, hay is generally thought of as a low quality protein source containing fewer of the essential amino acids per unit mass. Therefore to meet the horse’s essential amino acid requirements a greater amount of crude protein needs to be eaten. So excess protein may insure these amino acid needs are met. However much of the non-essential amino acids will need to be excreted and this excess nitrogen increases urea output which can lead to high ammonia levels in stalls and reduced respiratory health. This along with the fact that eating more hay may not be physically possible makes feeding a supplement or concentrate that provides essential amino acids an attractive alternative.

Fowden, A.L., A.J. Forehead, K.L. White and P.M. Taylor. (2000). Equine uteroplacental metabolism at mid and late gestation. Exp. Physiol. 85:539-545.

Greiwe-Crandell K.M., D.S. Kronfeld, L.A. Gay and D. Sklaty (1995). Seasonal Vitamin A Depletion in Grazing Horses Is Assessed Better by the Relative Dose Response Test than by Serum Retinol Concentration. J. Nutr. 125:2711-2716.

Lewis, L.D. Equine clinical nutrition: feeding and care. Philadelphia.Williams and Wilkins 1995.

National Research Council. Nutrient requirements of horses: 6^th Revised Edition. Washington D.C. National Academies Press. 2007.

Reynolds, L.P., C.l. Ferrel, D.A. Robertson, and S.P. Ford. (1986). Matbolism of the gravid uterus, foetus and utero-placenta at several stages of gestation in cows. J. Agric. Sci. 106:437-442.

Van Neikirk, F.E., and C.H. van Neikirk. (1997a). The effect of dietary protein on reproduction in the mare: III. Ovarian and uterine changes during the anovulatory, transitional and ovulatory periods in the non-pregnant mare. J.S. Afr. Vet. Assoc. 68:86-92.

Van Neikirk, F.E., and C.H. van Neikirk. (1997b). The effect of dietary protein on reproduction in the mare: I. The composition and evaluation of digestibility of dietary protein from different sources. J.S. Afr. Vet. Assoc. 68:78-80.

Feeding the 3rd Trimester Mare, Part 1 by Dr. Clair Thunes is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.