Supporting Your Mare and Foal: Essential Minerals for Optimal Health

As your mare approaches her foaling date, it’s an exciting period filled with anticipation as both she and the foal grow rapidly. The last trimester, covering the 9  8th to 11th months of pregnancy, is crucial as the foal’s growth accelerates, gaining up to 500 grams daily and reaching 60% of its birth weight. During these final months, the foal deposits most of its bone calcium and builds liver stores of essential minerals for post-birth growth. Since these minerals are low in mare’s milk, adequate liver reserves are vital to prevent developmental orthopaedic diseases (DOD) in young horses. Minerals are essential during growth, reproduction, and lactation. To support the rapidly growing foetus, the mare’s calcium and mineral needs increase, making it crucial to provide sufficient minerals.

Good pasture is the primary mineral source for late-pregnant mares, but nutrient levels can vary seasonally and annually. Research from the early 1980s indicated that mares in Victoria had insufficient calcium intake during Winter and Spring due to a lack of calcium-rich legumes in pastures. In the last three months of pregnancy and during lactation, forages are typically lower in phosphorus, and grass forages often do not meet the mare’s calcium and trace mineral needs. While hays generally contain more calcium than phosphorus, mineral levels can vary significantly between hay crops. Drought-affected pastures are particularly poor in minerals, necessitating additional calcium, magnesium, phosphorus, protein and energy to support pregnancy and lactation.

 Calcium and magnesium have fundamental roles in the development of bone and cartilage, necessitating increased dietary intake for mares to support the foetus’s bone growth.

CALCIUM
During pregnancy, mares allocate calcium to the growing foal, leading to lower blood calcium levels compared to non-pregnant mares. In late pregnancy, mares rely on skeletal calcium reserves, regulated by parathyroid hormone (PTH), which maintains blood calcium levels. Low calcium intake during late pregnancy and lactation increases PTH production, resulting in lower blood calcium and reduced bone density. Mares on higher dietary calcium have lower PTH levels, indicating less calcium removal from bones.

Bone density decreases in the first three months of lactation but can be restored by six months post-foaling with adequate calcium intake. However, mares with insufficient calcium intake may not regain bone density even 10 months post-foaling, and their foals may have weaker bones. Mares on oxalate-rich grasses (Table 1.) are at significantly higher risk of dietary calcium deficiency which is associated with retained membranes, foaling difficulties and fractures. The ratio of calcium : oxalate is important as it affects how much calcium can be absorbed from the pasture. A diet analysis can indicate the oxalate and calcium intake.

MAGNESIUM
Magnesium demand increases during pregnancy and lactation. Pregnant and lactating mares should receive 10-15g of Mg daily, with the NRC 2007 recommending 15g for a 500kg horse. Magnesium deficiency is linked to bone diseases in growing horses. Supplementing foals with 4g of Mg from birth to 12 months reduced Osteochondritis (OC) by 50% in the hock and fetlock at five months and by 14% in the stifle at 12 months of age. 

THE RISK OF DEFICIENCIES
Calcium and magnesium deficiencies are associated with various conditions in pregnant and lactating mares, including colic and retained placenta. Broodmares are at increased risk of colic from seven days before to 90 days after foaling. Low levels of these minerals can alter gut motility. Mares with post-foaling colic showed signs of dietary energy deficiency and lower blood calcium for two to four weeks before and after foaling.

Low blood calcium and magnesium can occur in transported horses and pregnant and lactating mares. While mares are less prone to ‘milk fever’ compared to other species, it can still occur, especially around 10 days after foaling or a few days after weaning. This condition has been reported in various breeds and situations. Signs of low blood calcium and/or magnesium include partially closed eyes, low head carriage, drooping bottom lip, restlessness, nervousness, sweating, rapid breathing, staring eyes, twitching muscles, depression, and abdominal pain. Urgent veterinary attention is required, and most cases respond to intravenous calcium and magnesium.

KEY TAKAWAY
It’s vital that a mare’s daily nutrient intake matches the nutrients she provides through her milk. If this balance isn’t maintained, the mare will deplete her own body reserves.