Equine Reproduction — Pre-Foaling Hay Nutrition and Safety

Üreme amaçlı kısraklar için saman: Doğum öncesi beslenme ve güvenlik

The hay a mare eats in the last 90 days before foaling determines colostrum quality, retained placenta risk, and whether she produces milk. Two factors dominate: potassium must stay below 1.5% DM in the final 30 days — a threshold most alfalfa hay exceeds — and fescue must be withdrawn 60–90 days before foaling. This guide covers the K-Ca mechanism, the species traffic light by trimester, and how to test and produce low-K hay for the broodmare market.

See Species Safety Guide

Why the Last 90 Days Change Everything in Mare Hay Management

Through most of a mare’s 11-month gestation, hay management follows the same general principles as any mature horse at light work — adequate protein, reasonable forage quality, and balanced minerals. The last 90 days before foaling are fundamentally different. The fetal growth curve is not linear: approximately 65–70% of the foal’s total birth weight accumulates in the final trimester, creating a period of exponentially rising nutritional demand. Simultaneously, the mare’s endocrine system is preparing for parturition, colostrum production, and lactation in a process that is acutely sensitive to dietary cation balance — specifically, the ratio of potassium to other minerals in the diet. Getting hay selection wrong in this period does not produce a minor quality difference; it can produce a mare with no milk, a retained placenta requiring emergency veterinary intervention, or a foal with inadequate passive immunity.

$800–$2,500
Typical veterinary cost for treating a retained placenta with associated endometritis and laminitis complications in a mare — a complication that is significantly more common in mares fed ergovaline-containing fescue hay or high-potassium hay in the final trimester
<1.5%
Target dietary potassium (K) as a percentage of diet DM in the last 30 days before expected foaling — a threshold that most pure alfalfa hay exceeds (alfalfa typically 1.8–2.8% K), making it important to test and manage K from hay in the late gestation period
60-90 gün
Minimum withdrawal period required from all tall fescue hay and pasture before the expected foaling date to eliminate the ergovaline-prolactin suppression effect that causes agalactia, retained placenta, and weak foal syndrome
This article provides nutritional information for hay management decisions — it does not replace veterinary and equine nutritionist guidance. Pre-foaling mare nutrition is highly individual: a mare’s history, body condition score, current diet composition, health status, and foaling date all affect the specific recommendations appropriate for her. The principles and thresholds in this guide represent current best practices from equine nutrition research literature, but the specific management plan for any broodmare should be developed in consultation with a veterinarian or equine nutritionist who knows the individual animal.

The Potassium Problem: Why High-K Hay Is Dangerous Close to Foaling

round baler producing hay in timothy grass field — timothy hay is one of the preferred pre-foaling hay choices for breeding mares because its typical potassium content of 0.8 to 1.5 percent falls within or near the recommended target for late-gestation mares, whereas alfalfa hay at 1.8 to 2.8 percent potassium frequently exceeds the threshold that increases hypocalcemia risk close to foaling

The connection between dietary potassium and pre-foaling hypocalcemia (low blood calcium) in mares is one of the most practically important and least widely understood concepts in equine pre-foaling nutrition. The mechanism involves the DCAD — dietary cation-anion difference — which quantifies the balance between positively charged dietary minerals (primarily sodium, potassium) and negatively charged minerals (primarily chloride, sulfur). When the DCAD is strongly positive (high potassium and sodium relative to chloride and sulfur), the body’s acid-base regulatory system responds in ways that impair the hormonal mobilization of calcium from bones during the critical pre-foaling period.

The mechanism — why high K suppresses calcium mobilization

In the days before foaling, the mare’s parathyroid hormone (PTH) must trigger release of calcium from bone reserves to supply the enormous calcium demand of colostrum production. A strongly positive DCAD (from high-K hay) shifts the body slightly toward metabolic alkalosis — an alkaline state that blunts the tissue response to PTH. Calcium receptors respond less efficiently to the PTH signal, and the mare cannot mobilize her bone calcium reserves at the rate colostrum demands. The result: blood calcium drops (hypocalcemia), muscle weakness develops, colostrum calcium concentration falls, and in severe cases the mare cannot stand or nurse the foal. The same mechanism causes “grass tetany” in cattle consuming high-K spring grass, and “milk fever” in high-producing dairy cows — the equine version is less frequently recognized but clinically significant in broodmares.

Potassium content of common hay types
Alfalfa hay: 1.8–2.8% K — typically exceeds the 1.5% target when fed as sole hay
Red clover hay: 2.0–3.0% K — very high; avoid in late gestation
Orchardgrass hay: 1.0–2.0% K (highly variable; test specific lot)
Timothy hay: 0.8–1.5% K — usually within or near target
Teff grass hay: 0.8–1.4% K — consistently low; excellent choice
Bermudagrass hay: 0.9–1.8% K (variable; test)
Native grass hay: 0.6–1.2% K — typically low
Alfalfa-grass 50/50 mix: 1.3–2.0% K — test specific lot
The anionic supplement option: When low-K hay is not available for the last 30 days before foaling, anionic salts (ammonium chloride, calcium chloride, magnesium sulfate) can be added to the diet to shift the DCAD toward negative — partially offsetting the alkalosis-promoting effect of high-K hay. This is used routinely in pre-calving dairy cow programs and is increasingly used by equine nutritionists in Thoroughbred and performance horse breeding programs. Palatability is the primary management challenge: anionic salts are bitter and mares often reduce intake when they are added to feed. If you are considering anionic supplementation for a mare on high-K hay, consult with an equine nutritionist or veterinarian for appropriate dosing and palatability management — incorrect dosing produces metabolic acidosis, which is as harmful as the alkalosis it is intended to prevent.

Fescue Hay and Pregnant Mares: A Non-Negotiable Withdrawal Requirement

Tall fescue hay from stands infected with the toxic endophyte (Epichloë coenophiala) is one of the most well-documented reproductive hazards in equine management. The ergovaline produced by the endophyte suppresses prolactin — the hormone that triggers milk production, colostrum secretion, and several processes involved in normal parturition. The consequences for mares fed toxic fescue in late gestation are clinically serious and frequently require emergency veterinary intervention. University of Kentucky and other equine research programs have documented these outcomes consistently across multiple study populations.

AGALACTIA
No milk
Complete or near-complete failure to produce colostrum or milk at foaling. Ergovaline suppresses prolactin production — the hormone that signals the mammary gland to initiate milk and colostrum secretion. A foal born to an agalactic mare receives inadequate immunoglobulins (IgG) from colostrum, leaving it vulnerable to septicemia, respiratory infections, and other neonatal conditions. Emergency response: colostrum from a donor mare or commercial equine colostrum bank must be administered within the first 6–12 hours of life. Cost of emergency colostrum sourcing: $150–$400 in most regions.
RETAINED PLACENTA
Failure of the placenta to be expelled within 3 hours of foaling. Research from University of Kentucky documented retained placenta rates of 35–50% in mares on toxic fescue versus 2–5% in control mares on clean forage. Retained placenta leads to endometritis (uterine infection), systemic toxemia, and — as a secondary consequence — laminitis from endotoxin absorption. This is a veterinary emergency requiring oxytocin treatment, manual extraction assistance, systemic antibiotics, and anti-inflammatory medication. Total treatment cost: $500–$1,500 minimum.
PROLONGED GESTATION
Gestation extended 10–20+ days beyond normal (320–360 days). Post-mature foals from prolonged gestation are often oversized and physically immature (paradoxically) — with reduced joint flexibility, poor suckle reflex, and neurological immaturity (weak foal syndrome). The extended gestation also significantly increases the probability of dystocia requiring obstetric intervention.
The 60–90 day withdrawal rule — why timing matters

Ergovaline’s effect on prolactin is not immediate — it accumulates over weeks of exposure and clears slowly after the source is removed. Removing a mare from fescue hay 60 days before the expected foaling date provides adequate time for ergovaline to clear from the system and prolactin levels to normalize before colostrum production begins. Most equine veterinary specialists recommend 90 days as a more conservative margin for mares with a history of fescue-related problems, mares over 15 years old, or mares in their first foaling. The withdrawal applies to both fescue hay AND fescue pasture — both sources deliver ergovaline at significant concentrations.

Novel endophyte fescue — cautious approach recommended

Novel endophyte fescue varieties (MaxQ and others) produce no ergovaline and have not been shown to cause the classic reproductive complications associated with toxic endophyte fescue in controlled studies. However, most equine veterinary specialists recommend maintaining a 60-day withdrawal from all fescue hay — including novel endophyte varieties — as a precautionary measure close to foaling. The reasoning: the reproductive stakes are high, the withdrawal has no cost in a hay that has clean alternatives, and the margin of confidence in any novel endophyte-derived hay recommendation is not absolute. Consult your veterinarian for guidance specific to your mares’ history and your hay supply situation.

Hay Species Safety by Trimester: The Traffic Light Guide

Not all hay recommendations for mares are static across the full 11-month gestation. The K concern is primarily a late-gestation issue; fescue must be avoided throughout; certain legume-quality advantages apply more in early gestation than late. This trimester-organized guide gives specific guidance for each of the most commonly available hay species.

Hay species Early–Mid gestation
Months 1–7		 Late gestation
Months 8–10		 Last 30 days
Pre-foaling		 Lactation
Post-foaling
Timothy hay ✓ SAFE ✓ SAFE ✓ SAFE (test K) ✓ SAFE
Teff grass hay ✓ SAFE ✓ SAFE ✓ PREFERRED ✓ SAFE
Bermudagrass hay ✓ SAFE ✓ SAFE ⚠ TEST K ✓ SAFE
Orchardgrass hay ✓ SAFE ✓ SAFE ⚠ MUST TEST K ✓ SAFE
Alfalfa hay ✓ SAFE ⚠ MONITOR K ⚠ TEST — limit or blend ✓ EXCELLENT
Native grass hay ✓ SAFE ✓ SAFE ✓ SAFE ⚠ Supplement protein
Red clover hay ⚠ CAUTION (slaframine) ✗ AVOID (high K) ✗ AVOID ⚠ Limited use
Toxic fescue (KY-31) ✗ AVOID all gestation ✗ AVOID ✗ ABSOLUTELY NOT ✗ AVOID (affects milk)

Alfalfa in Late Gestation: The Practical Balance

round baler detail showing key production features — producing documented low-potassium grass hay for the broodmare market requires the same equipment and production standards as any premium quality hay, but with the addition of mineral analysis including potassium content in the forage test; the value premium for tested low-K hay in broodmare markets is $20 to 40 per ton above undocumented equivalent quality hay

The alfalfa-in-late-gestation question creates genuine uncertainty among mare owners and hay producers because the answer is nuanced. Alfalfa is an excellent mare hay in most of gestation and an excellent lactation hay — its calcium content (1.2–2.0%) supports the mare’s bone calcium mobilization and colostrum production, and its protein (18–24% CP) supports fetal growth. The specific concern is the potassium content and the last 30 days before foaling, when the DCAD effect on calcium mobilization is most critical.

The practical resolution by phase
Months 1–7 (early/mid gestation): Alfalfa is appropriate as a primary or partial hay source. No K concern at this stage. CP and Ca support early fetal development.
Months 8–10 (late gestation): Transition from pure alfalfa to a 50/50 alfalfa-grass mix. A well-managed blend typically tests 1.3–1.8% K — within or near the acceptable range. Test the specific mix’s K before relying on it.
Last 30 days (pre-foaling): Pure grass hay (timothy, teff, tested low-K orchardgrass) is preferable if available. If the alfalfa-grass blend tests below 1.5% K, it can continue under veterinary guidance. Pure alfalfa at 2.0–2.8% K is generally not recommended as the sole hay for the final 30 days.
Alfalfa post-foaling: excellent choice

Once the mare has foaled, the K restriction concern diminishes significantly. The lactating mare’s tremendous energy and calcium requirements make alfalfa an excellent hay choice post-foaling — its high CP supports milk protein, its high calcium supports the 4–6 g of calcium per liter of milk the mare produces, and its high energy density helps the mare maintain body condition through the lactation period. Operations that keep mares on grass hay post-foaling for extended periods often see lactating mares lose condition because they cannot consume sufficient grass hay to meet the caloric demands of peak milk production. Transitioning back to alfalfa-grass mix or pure alfalfa within the first week post-foaling is appropriate management for most broodmares.

Forage Testing for Mare Nutrition: What to Order and When

The standard forage test ordered for cattle hay — CP, ADF, NDF, TDN — is insufficient for broodmare hay management because it does not include the mineral values that determine late-gestation hay safety. Potassium must be specifically requested; it is not included in any standard panel offered by NFTA-certified laboratories. The additional mineral tests add approximately $15–$25 to a standard panel and provide information that is essential for the last-90-days management period.

Minimum panel for broodmare hay
  • Dry matter and moisture
  • Crude protein (CP)
  • ADF and NDF
  • Calcium (Ca%) — must be specifically requested
  • Potassium (K%) — must be specifically requested
  • Phosphorus (P%) — for Ca:P balance
  • Magnesium (Mg%) — relevant to K interaction
When to test

Test every new hay lot, not just once per season. Potassium content varies significantly between cuttings — spring hay often tests 20–40% higher K than fall hay from the same field because of spring luxury K uptake when soil K and soil moisture are both high. A fall-cut timothy lot that tested 1.1% K in October may not be representative of the same farm’s May-cut timothy. Test each lot before committing it as the primary hay for the final trimester. For the complete horse hay quality testing framework, see the At yemi kalitesi ve NSC spesifikasyonları kılavuzu.

Quality targets by gestation stage
Early-mid gestation: CP 10–12%; NDF 50–60%; maintenance quality acceptable
Last trimester: CP 12–14%; K <1.8% (ideally <1.5% for last 30 days); Ca 0.5–0.8%
Lactation: CP 14–16%; Ca 0.6–0.9%; energy-dense; K restriction less critical

Producing Low-K Hay: The Premium Broodmare Market Opportunity

foragebaler.com quality commitment and equipment standards — producing documented low-potassium hay for breeding mare operations requires the same equipment standards as any premium hay production, but adds a mineral analysis step that differentiates the product in a growing specialty market; thoroughbred breeding farms and premium equine operations increasingly specify potassium content along with NSC and protein when purchasing hay for broodmare programs

A small but growing hay market segment specifically requests documented low-K hay for broodmare programs — Thoroughbred breeding farms, warmblood sport horse breeding operations, and premium equine boarding stables with active broodmare programs. These buyers pay a $20–$40/ton premium for hay with documented mineral analysis confirming K below 1.5%, because the cost of one retained placenta or agalactia event exceeds the entire season’s hay premium many times over. For hay producers in regions with significant equine breeding operations, understanding and producing for this market is a meaningful revenue opportunity.

Production practices that reduce hay K

Soil potassium management is the primary lever for hay K content. Fields with historically high potassium fertility (from heavy manure application, legacy K fertilization, or high-K parent material) produce hay with consistently high K regardless of species. To produce reliably low-K hay: (1) Test soil K; target fields with lower available soil K; (2) Do not apply potassium fertilizer to fields used for broodmare hay; (3) Allow soil K to deplete through crop removal without replacement over 2–3 seasons; (4) Harvest late summer or fall cuts rather than spring cuts — fall hay typically tests 20–35% lower K than spring hay from the same field due to reduced luxury uptake. The teff grass and timothy production guides cover the species-specific production practices for the two most consistently low-K hay types: teff grass hay production guide Ve timothy hay production and baling guide.

Marketing and documentation approach

Market low-K hay directly to breeding farms and equine nutritionists — these buyers understand the K:Ca management issue and actively seek documented hay. Provide the full mineral panel analysis (CP, K, Ca, P, Mg, NSC for horse-appropriate species) with every delivery; buyers at this tier will not purchase hay without documentation. Price the premium at $25–$40/ton above equivalent undocumented quality timothy or teff. Build the documentation package: soil test confirming low K fertilization history; cutting date and timing (fall vs spring); full forage mineral panel. The yuvarlak balya makinesi modelleri suited to producing consistent, well-conditioned timothy and teff bales for premium horse markets are available through our product line. Consistent bale density for even curing — important for producing hay below 14% moisture for the horse market — requires the appropriate density spring setting and PTO specifications per tarımsal şanzıman ve PTO tahrik sistemi bileşenlerinin özellikleri.

Hay for Breeding Mares FAQs

What is the safest hay choice for a mare in the last month before foaling?+
Teff grass hay and timothy hay are the two most consistently recommended hay types for the final 30 days before foaling because both typically test below 1.5% potassium — the threshold that most equine nutritionists target for late-gestation K management. Teff has the advantage of being consistently low in both K and NSC (suitable also for insulin-dysregulated mares), while timothy’s well-recognized palatability and widespread horse-market acceptance make it the more commonly available premium option in most regions. Either hay should be accompanied by a current mineral analysis confirming K content for the specific lot being fed — “teff” or “timothy” as a species designation is not sufficient; K varies by cutting, soil, and season, so lot-specific testing is essential. If only grass-legume mixed hay is available in your area, have the specific lot tested; a blend that tests below 1.5% K total is acceptable. If you are uncertain about the appropriate hay choice for your mare’s specific situation, consult with your equine veterinarian or a qualified equine nutritionist before the last-30-days period — that is not the time to make unguided dietary changes.
Can I feed alfalfa hay to a pregnant mare?+
Yes — with stage-specific modifications. Alfalfa is an excellent hay for mares through the first 7–8 months of gestation; its high protein (18–24% CP) and high calcium support fetal development and maternal bone reserves. In months 8–9, transitioning from pure alfalfa to a 50/50 alfalfa-grass mix is a prudent step to begin managing the dietary K loading. In the final 30 days before foaling, pure alfalfa is not recommended as the primary hay source unless the specific lot has been tested and confirmed below 1.5% K DM basis — a threshold most alfalfa lots will not meet. The practical path used by most breeding farms: pure alfalfa or alfalfa-dominant hay through mid-gestation; 50/50 mix with K monitoring in the last trimester; grass hay (timothy or teff) as primary hay in the last 30 days with alfalfa available in smaller amounts as a supplemental protein and energy source if the mare needs additional body condition. After foaling, alfalfa is an excellent primary hay for the lactating mare — the K concern largely resolves post-foaling, and the high calcium and protein make alfalfa ideal for supporting milk production.
How do I test my hay’s potassium content for broodmare management?+
Submit a hay sample to any NFTA (National Forage Testing Association) certified laboratory and specifically request potassium (K%) along with the standard analysis panel. The laboratory order form will have a mineral analysis or “complete mineral panel” section — make sure K is included. This is not included in the standard CP/ADF/NDF panel on any laboratory’s price list, so it must be specifically requested. Additional minerals worth adding for complete broodmare hay management: calcium (Ca%), phosphorus (P%), and magnesium (Mg%); these together with K provide the full mineral picture needed for DCAD assessment. Cost of the mineral add-on: $15–$25 above the standard panel, depending on the laboratory. Sampling protocol: use a hay core sampler to collect samples from at least 10 bales per lot; combine all core samples into one composite sample bag; submit the composite. This gives a representative lot average rather than a single-bale outlier. The mineral panel result for K and the decision of whether it falls within an acceptable range for your specific mare should be reviewed with your equine veterinarian or nutritionist, particularly for mares with a history of milk fever, retained placenta, or other reproductive complications.
My mare foaled without milk — could the hay have caused it?+
Agalactia (absence of milk at foaling) in a mare has several possible causes, and two of the most common are directly hay-related: fescue toxicosis and hypocalcemia from high-K diet. If the mare was on fescue hay or fescue pasture in the 60–90 days before foaling, ergovaline-induced prolactin suppression is the most likely explanation. If the mare was on primarily alfalfa or high-K hay (K above 1.5% on a DM basis) in the final 30 days, hypocalcemia-related agalactia is a possibility. In either case, the immediate priority is the foal — a foal without colostrum in the first 6–12 hours of life is at risk for failure of passive transfer (FPT) of immunity. Contact your veterinarian immediately; emergency colostrum from a donor mare, commercial equine IgG supplement, or plasma transfusion may be required. After addressing the foal’s immediate needs, have the mare evaluated by your veterinarian for hypocalcemia (blood calcium test) and review the hay feeding history for both fescue exposure and K loading. Going forward, changing hay type before the next foaling — specifically testing K content and confirming fescue-free sourcing — is the appropriate preventive measure. Work with your equine veterinarian to develop a specific pre-foaling nutrition protocol for the mare’s next pregnancy.
How much hay does a late-gestation mare need per day?+
A late-gestation mare (last 90 days) requires approximately 2.0–2.5% of her body weight in total dry matter daily — approximately 20–25 lbs/day for a 1,000-lb mare. At this intake level with good-quality hay (12–14% CP, 0.5–0.8% Ca), most mares can meet their nutritional requirements from hay alone without grain supplementation, unless the mare is thin (body condition score below 5/9) or carrying twins. The common mistake: reducing hay feeding in late gestation because “the mare looks full” from the large fetal mass. The mare’s digestive tract is compressed by the growing fetus, reducing rumen volume — she will eat smaller meals but needs more frequent access to hay rather than reduced total access. Provide hay free-choice or in multiple daily feedings rather than one or two large meals in the last 4–6 weeks of gestation. Body condition score should be 5.5–6.5 on a 9-point scale at foaling time — thin mares have worse colostrum quality and poorer lactation performance; over-conditioned mares have higher dystocia risk. Adjust grain supplementation — not hay intake — if fine-tuning body condition in the last trimester, as grain supplementation allows more precise energy dosing than adjusting hay volume.
Can I use the same hay for all horses in a mixed herd that includes pregnant mares?+
For most of the year, yes — a quality mixed grass-legume hay that serves geldings, performance horses, and non-pregnant mares can also serve pregnant mares through mid-gestation without specific concern. The challenge arises in the last 90 days before foaling, when the K management requirement for the late-gestation mare begins to diverge from the needs of the rest of the herd. The practical solution used by most facilities: maintain the standard herd hay for non-pregnant horses and maintain a separate, specifically selected hay (lower-K, fescue-free, tested) for mares in the last trimester. This requires separate feeding for the late-gestation mares — which most facilities already do for other management reasons (body condition management, supplementation). If separate feeding is not practical due to pasture or stabling configurations, choosing a herd hay that meets the broodmare requirements (timothy or teff, tested K below 1.5%, no fescue) provides safety for the mares while being nutritionally adequate for the rest of the herd — the non-pregnant horses simply receive a lower-K hay than is strictly necessary for them, which causes no harm.
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