Essential Nutrients for Broodmares: A Complete Guide – Part 4

Welcome to Part 4 of our broodmare nutrition series! Now that we’ve covered feeding strategies for each stage of reproduction, let’s dive deep into the nutrients themselves. Understanding what each nutrient does and why it matters will help you make informed decisions about supplements, feed choices, and troubleshooting nutritional problems. This is the science behind the feeding strategies!

Energy: The Foundation of Everything

Energy, measured in digestible energy (DE) or megacalories (Mcal), provides the fuel for all body functions including maintenance, pregnancy, and lactation. Think of it as the gasoline that powers the engine—without adequate energy, nothing else works properly.

What Happens When Energy Is Wrong

Too little energy: The mare mobilizes fat reserves, then muscle tissue. She loses weight, milk production drops, and the foal doesn’t thrive. In severe cases, the mare may stop cycling entirely.

Too much energy: The mare becomes obese, leading to dystocia, metabolic disorders, reduced fertility, and developmental problems in offspring.

Best Energy Sources

• Forage: Hay and pasture provide the foundation. Quality grass hay provides 1.8-2.0 Mcal/kg, alfalfa 2.0-2.4 Mcal/kg
• Grains: Oats (3.0 Mcal/kg), corn (3.5 Mcal/kg), and barley (3.2 Mcal/kg) are concentrated energy sources
• Fats: Vegetable oils and rice bran provide 8-9 Mcal/kg—the most concentrated energy available

Pro tip: Always build energy intake from forage first, then add concentrates as needed. A forage-first approach supports digestive health and prevents behavioral issues.

Protein: Quality Matters Most

While crude protein percentages are commonly discussed, what truly matters is the quality and amino acid composition of that protein. The horse cannot synthesize certain essential amino acids and must obtain them from the diet.

Lysine: The First Limiting Amino Acid

Of all the essential amino acids, lysine is typically the first limiting amino acid in equine nutrition. This means it becomes deficient before other amino acids if protein quality is poor. During pregnancy, protein supports fetal tissue development. During lactation, it’s essential for milk protein synthesis.

Daily lysine requirements:

• Maintenance: 20-25 grams
• Late pregnancy: 30-35 grams
• Peak lactation: 40-50 grams

Best Protein Sources

• Soybean meal: Excellent amino acid profile, 44-48% protein, high lysine content
• Alfalfa: Good quality protein, 15-20% protein, readily available
• Commercial feeds: Quality brands balance multiple protein sources for optimal amino acid profiles

What to avoid: Poor-quality protein sources like low-grade supplements don’t provide adequate amino acids for reproduction, regardless of crude protein percentages.

Calcium and Phosphorus: The Skeletal Framework

These macrominerals work together to build and maintain the skeletal system. The developing fetal skeleton and the mineral content of milk create high demands for both minerals, particularly during late pregnancy and lactation.

The Critical Ratio

Beyond absolute amounts, the ratio of calcium to phosphorus is critical. The ideal ratio is approximately 2:1, though ratios anywhere from 1.2:1 to 6:1 are tolerable if both minerals are provided in adequate amounts.

Why ratios matter: Excess phosphorus relative to calcium interferes with calcium absorption and can lead to nutritional secondary hyperparathyroidism, even when calcium intake appears adequate. This condition weakens bones and can cause developmental problems.

Daily Requirements

Requirements increase dramatically through reproduction:

• Early pregnancy: 30g calcium, 20g phosphorus
• Late pregnancy: 40-50g calcium, 25-30g phosphorus
• Peak lactation: 45-65g calcium, 30-40g phosphorus

Natural Sources

High in calcium: Alfalfa hay (1.2-1.5% calcium), legume hays, limestone

High in phosphorus: Grains (oats, corn, barley contain 0.3-0.4% phosphorus), wheat bran

Balanced approach: Mix alfalfa with grass hay and balance grain-heavy diets with calcium supplementation or alfalfa.

Trace Minerals: Small Amounts, Huge Impact

Although required in much smaller quantities than macrominerals, trace minerals are equally essential for health and development. Deficiencies can have devastating long-term effects.

Copper: The DOD Fighter

Copper is essential for proper bone and cartilage formation. Copper deficiency is strongly associated with developmental orthopedic disease (DOD) in growing foals—a collection of conditions affecting bone and cartilage development that can cause lameness and reduced performance.

Functions: Bone formation, connective tissue development, coat color (melanin production), immune function

Daily requirements: 100-125 mg early pregnancy, 125-250 mg late pregnancy and lactation

Problem: Many regions have copper-deficient soils, making supplementation essential. Most pasture and hay provide inadequate copper.

Zinc: Copper’s Partner

Working closely with copper, zinc is vital for skeletal development, skin and hoof integrity, wound healing, and immune function. The two minerals have an important relationship—they compete for absorption, so the ratio matters.

Daily requirements: 400-500 mg early pregnancy, 500-700 mg late pregnancy and lactation

Optimal ratio: Copper to zinc should be approximately 1:3 to 1:4 (e.g., if providing 200mg copper, provide 600-800mg zinc)

Selenium: The Antioxidant Mineral

This antioxidant mineral works synergistically with vitamin E to protect cells from oxidative damage. Selenium is critical for immune function, fertility, and muscle development.

Daily requirements: 1-3 mg depending on regional levels

Critical warning: Selenium has a narrow margin of safety between deficiency and toxicity. Total intake should not exceed 3 mg daily. Check if your commercial feed already contains selenium before adding supplements.

Manganese: The Overlooked Mineral

Important for bone development, carbohydrate metabolism, and reproduction, manganese works with other trace minerals to support skeletal health. While less discussed than copper and zinc, manganese deficiency can contribute to reproductive problems and skeletal abnormalities.

Daily requirements: 400-500 mg for broodmares

Vitamins: Essential Catalysts

Vitamin A: Vision and Immunity

Essential for vision, immune function, growth, and reproduction, vitamin A is abundant in green, growing pasture but degrades rapidly in stored hay. Mares on hay-only diets almost always require supplementation.

Daily requirements: 30,000-45,000 IU for broodmares

Signs of deficiency: Night blindness, reproductive problems, increased infection susceptibility, poor hoof quality

Vitamin D: The Calcium Manager

Necessary for calcium absorption and bone mineralization, horses synthesize vitamin D from sunlight exposure. Horses with outdoor access typically don’t require supplementation, but stall-kept horses may benefit from 3,000-6,000 IU daily.

Vitamin E: The Master Antioxidant

A powerful antioxidant that protects cell membranes from oxidative damage, vitamin E is crucial for immune function, muscle integrity, and neurological health. Fresh pasture is the best natural source, but vitamin E degrades rapidly in hay—within weeks of cutting.

Daily requirements: 1,000-2,000 IU for pregnant mares, 2,000-3,000 IU during late pregnancy and lactation

Important: If your mare is on hay with no pasture access, vitamin E supplementation is essential. Natural vitamin E (d-alpha-tocopherol) is better absorbed than synthetic forms.

Water: The Most Critical Nutrient

Water is perhaps the most critical nutrient, yet it’s often overlooked in discussions of broodmare nutrition. Lactating mares have enormous water requirements—potentially 50-70 liters (13-18 gallons) daily in warm weather or during heavy milk production.

Why Water Matters So Much

• Milk is 90% water—inadequate water rapidly reduces milk production
• Dehydration increases colic risk, especially impaction colic
• Water regulates body temperature during lactation’s increased metabolic demands
• Proper hydration ensures optimal nutrient absorption

Ensuring Adequate Water Intake

• Provide constant access to clean, fresh water
• In winter, use heated water sources to encourage drinking
• Clean water buckets and automatic waterers daily
• Monitor water quality—high mineral content or contamination reduces intake
• Watch for ice formation in winter that may block access

Putting It All Together

Understanding these nutrients helps explain why proper broodmare nutrition requires careful attention to detail. It’s not just about feeding more during lactation—it’s about ensuring the right balance of energy, protein quality, mineral ratios, and vitamin supplementation throughout the reproductive cycle.

In our final installment, Part 5, we’ll bring everything together with practical feeding strategies, common mistakes to avoid, and troubleshooting tips for when things don’t go as planned.

Complete Series Now Available

Download our comprehensive Broodmare Nutrition Guide (PDF) containing all five parts of this series plus feeding charts, supplement guides, and troubleshooting resources. Free download!

Review the Complete Series:

• Part 1: Broodmare Nutrition Basics
• Part 2: Late Pregnancy Nutrition
• Part 3: Feeding the Lactating Mare
• Part 4: Essential Nutrients for Broodmares (You are here)
• Part 5: Practical Strategies & Common Mistakes

Further Reading

• NRC (National Research Council) – Nutrient Requirements of Horses, 6th Revised Edition
• American Association of Equine Practitioners resources on broodmare care
• University extension publications from equine programs (Kentucky, Virginia Tech, UC Davis)

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