Feeding the PSSM horse

A practical protocol for the horse owner who has a confirmed PSSM diagnosis and wants to know what to actually do this week. Backed by 11 systematic reviews. Pair this article with the Hay NSC Calculator and Daily Ration Builder tools.

The four numbers that matter

Step 1: Test your hay

You cannot tell hay's NSC by looking at it. Two horses on visually identical hay can have wildly different blood-sugar responses. Forage testing is the foundation of PSSM management[7].

Two industry-standard US labs:

• Equi-Analytical Laboratories (Ithaca, NY), equine-specialized, well-validated.
• Dairy One Forage Lab (Ithaca, NY), broader livestock focus, also equine-capable.

Cost: $25-$70 per sample depending on the panel. Turnaround: 5-10 business days for wet chemistry. Sample protocol matters: take core samples from at least 12 bales using a hay probe, mix, and ship the composite. A single grab from one bale tells you almost nothing[7].

What to look for on the report: NSC = WSC + starch (water-soluble carbs plus starch). Some labs report ESC (ethanol-soluble carbs, mostly simple sugars). Aim for total NSC under 10-12%. ESC is a tighter measure for insulin-resistant or severe PSSM2 horses.

Step 2: Pick a base feed strategy

Three viable strategies, depending on your horse's energy needs:

A. Forage + ration balancer (low-energy horses)

Easy keepers, retired horses, light work. Tested low-NSC forage as the foundation, plus a low-volume ration balancer to fill mineral and vitamin gaps. Often this alone is sufficient.

B. Forage + ration balancer + oil (moderate-energy horses)

Schooling horses, light competition. Add 1-2 cups of oil (canola, flaxseed, or rice bran oil) per day to push fat percentage and provide concentrated calories without carbohydrate. Introduce gradually over 2-3 weeks to allow adaptation.

C. Forage + commercial PSSM-formulated feed (high-energy horses)

Active competition, heavy work, hard keepers. A commercial low-NSC + high-fat feed, fed at the manufacturer's recommended rate. See lit_review_18_commercial_feeds_claims for the claim-validation table; not all products marketed as "PSSM-suitable" pass independent scrutiny[18].

Step 3: Decode the feed tag

Required label elements vs. what you actually need to know:

If starch and NSC are not on the tag (and they often are not), email the manufacturer and ask. A reputable manufacturer will tell you. If they will not, that is information.

Step 4: Time the meals

Per lit_review_09_meal_timing_turnout, meal timing matters:

• Frequency: 3-4 small meals per day beats 2 large ones. Reduces glycemic spike, easier on the gut.
• Pre-exercise: Avoid feeding concentrate within 90 minutes before work. Fat-based meals are more tolerable than carbohydrate-based pre-work.
• Overnight fasting: Free-choice forage overnight. Long fasting periods promote stress and gastric ulceration[33].
• Pasture turnout timing: Pasture grass NSC peaks in afternoon and after frost. Limit turnout 10am-5pm during high-NSC seasons. Use grazing muzzle or dry-lot management for severely affected horses[9].

Step 5: Supplements (the short list with evidence)

What the research actually supports, in priority order:

1. Vitamin E (high priority). Most PSSM horses benefit from supplementation, particularly if forage is stored more than 6 months (vitamin E degrades). Natural d-alpha-tocopherol preferred when affordable[13].
2. Selenium (regional). Required at 0.1-0.3 mg/kg dry matter intake in selenium-deficient regions. Toxic above 5-10 mg/kg, so confirm regional status before supplementing[15].
3. Magnesium (case-by-case). Some horses with stress-related muscle tension benefit; others see no effect. 5-10 g elemental magnesium per day is a reasonable trial dose. Discontinue if no improvement after 60 days[19].
4. Omega-3 fatty acids (modest evidence). Anti-inflammatory effects shown in exercise physiology studies; specific PSSM benefit less well documented but plausible mechanism[14].

What the research does not robustly support, despite confident manufacturer claims:

• Branched-chain amino acid supplements specifically for PSSM
• "Antioxidant blends" beyond vitamin E + selenium
• Proprietary PSSM-specific formulas claiming benefit beyond their NSC and fat composition
• Specific commercial probiotic blends

None of these are necessarily harmful; they are simply not evidence-validated. See the supplement claim-validation table in lit_review_18_commercial_feeds_claims.

Step 6: Body condition and weight

The PSSM diet must match the horse's actual energy needs[16]:

• Easy keepers and overweight: Use forage-only or forage + ration balancer; monitor body condition score (BCS) and aim for BCS 5-6 on the Henneke scale. PSSM horses are not exempt from obesity-related insulin dysregulation[16].
• Thin or underweight: See lit_review_17_underweight_pssm. Calorie density must come from fat, not carbohydrate. Stabilized rice bran, fat supplements, and oils are the lever.
• Maintenance: Most PSSM horses on the standard protocol settle into BCS 5-6 with good muscle development if exercise is consistent.

Step 7: Monitor and adjust

Per lit_review_10_biomarkers_dietary_response, the standard monitoring panel:

• Resting CK and AST, baseline at start of diet, then 60 days later, then annually. Expect 70-90% reduction with full protocol adherence.
• BCS, monthly visual assessment. Easier to track with standardized photos.
• Tying-up episode log, record date, severity, weather, recent feed/work changes. Pattern recognition over 6-12 months is informative.
• Vitamin E level, annual blood test, particularly if hay is stored long-term or in regions with selenium-deficient soils.

What "good control" looks like

• Resting CK in the normal range (typically 100-300 U/L; lab-specific)
• Less than 2 episodes per year (down from monthly)
• Stable BCS 5-6, with improving muscle definition
• Consistent willingness to work; no sudden refusals or behavioral changes that suggest pain
• Stable performance trajectory in trained discipline

If you have followed this protocol consistently for 6 months and are not seeing this picture, your horse may have been misdiagnosed (RER, EPM masquerade, eNAD), may have a more severe presentation, or may have a co-occurring condition (gastric ulcer, hindgut acidosis, EPM). Bring it back to your vet with a complete log of what you have tried.

References (this article)

1. Forage testing methodology (lit_review_07_forage_testing_methodology)
2. Forage sampling protocols (lit_review_07_forage_testing_methodology)
3. Commercial feed claim validation (lit_review_18_commercial_feeds_claims)
4. GI comorbidity (lit_review_33_gi_comorbidity)
5. Meal timing and turnout (lit_review_09_meal_timing_turnout)
6. Antioxidants in PSSM (lit_review_13_antioxidants_pssm_epm)
7. Selenium in PSSM/EPM (lit_review_15_selenium_pssm_epm)
8. Micronutrients beyond E/Se (lit_review_19_micronutrients_beyond_E_Se)
9. Omega-3 in neuromuscular disease (lit_review_14_omega3_neuromuscular)
10. Body condition and PSSM feeding (lit_review_16_body_condition_insulin)
11. Insulin dysregulation in obesity (lit_review_16_body_condition_insulin)