Somatic Cell Count

What is Somatic Cell Count?

Somatic Cell Count (SCC) measures the number of white blood cells (leukocytes) per milliliter of milk. It is the gold standard indicator of udder health and the primary tool for detecting mastitis — both clinical and subclinical.

Normal milk from a healthy udder contains fewer than 100,000 cells/mL. When bacteria infect the udder, the cow's immune system sends white blood cells to fight the infection, causing SCC to rise. A bulk tank SCC above 200,000 cells/mL suggests significant subclinical mastitis in the herd.

SCC is reported as either individual cow SCC (from milk recording) or bulk tank SCC (from the cooperative or processor). Most cooperatives pay quality premiums for bulk tank SCC below 200,000 and impose penalties above 300,000–400,000.

The financial impact is significant: every 100,000 increase in bulk tank SCC above 200,000 costs approximately 2.5 kg of milk per cow per day. For a 200-cow herd, this translates to $5,000–$15,000 in annual losses from production reduction, penalties, treatment costs, and premature culling.

Somatic Cell Score (SCS) is a linear transformation of SCC using the formula SCS = log₂(SCC/100,000) + 3, converting the logarithmic scale to a more intuitive 0–9 range.

Understanding the Logarithmic Scale

SCC is reported on a logarithmic scale because cell counts vary exponentially. A bulk tank reading of 200,000 is not "twice as bad" as 100,000 — it represents a 10-fold increase in bacterial challenge. This is why the Somatic Cell Score (SCS) transformation is useful: SCS increases by 1 point for every doubling of SCC. SCS 2 = 50,000 SCC, SCS 3 = 100,000, SCS 4 = 200,000, SCS 5 = 400,000, SCS 6 = 800,000. Most genetic evaluations and breeding values use SCS rather than raw SCC.

SCC and Milk Quality Premiums

Cooperative payment structures typically follow this pattern: SCC below 100,000 earns a premium of $0.15–$0.30/cwt. SCC 100,000–200,000 earns base price. SCC 200,000–300,000 incurs a penalty of $0.10–$0.20/cwt. SCC 300,000–400,000 incurs a penalty of $0.20–$0.50/cwt. SCC above 400,000 may result in milk rejection. For a 200-cow herd producing 80 lbs/cow/day, the difference between SCC of 150,000 and 350,000 is approximately $8,000–$15,000/year in premiums lost and penalties incurred.

Seasonal SCC Patterns

SCC typically rises in summer due to heat stress (which suppresses immune function), increased environmental bacterial loads (warm, moist bedding), and flies serving as mechanical vectors for mastitis-causing bacteria. Winter SCC may also rise in confined systems with poor ventilation and wet bedding. Understanding seasonal patterns allows proactive management — increasing milking frequency, improving teat disinfection, and enhancing bedding hygiene before SCC spikes occur.