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Grow longhorn cows, after that collect their leather. If you want you can sell their leather straight away you can earn over 1 million equus. Use large meadows and LONGHORN cows very important. If you want even more profit you can produce Lunges with the leather you have ( large workshops can produce up to 6 lunges per session) The higher prestige your EC has the more workshops you can have( 5 is max). If you have 5 large workshops you can produce 30 lunges in 5 days ( 250,230 equus) every 5 days if you sell them all every 5 days. You can always stack them and later sell when you havce 100 200 lunges.

In addition: check this page https://www.howrse.com/centre/atelier/rentabilite
It's about the profitability of the products you make in workshops, and shows the sale value and the average profit per hour of production.

I'd go with growing cows then using the leather to make the bonus items. Also, EC center running western comps will give you leather for the cutting cows that get changed out. The lunge is pretty good money-wise though they do take some time to finish. Or, if your EC is in the mountains, you could have the horses run mission to gather iron and then you could make stall/field enhancements or spurs. ^^
TLDR: lunges if you're also growing cows and/or spurs if you're EC is in the mountains