One horse: 10 tons of manure per year. Two horses — 20 tons. Problem? No — resource.
Cycle
Manure → compost pile (6 months) → fertilizer → field → feed → horse → manure. Closed loop. Free. Forever.
Comparison
Chemical fertilizer: $200-500/acre/year. Horse manure: $0/acre/year. Over 20 years: $4,000-10,000 savings per field. And soil with manure is alive. With chemicals — dead in 30 years.