Butter is derived from the fat of milk and contains fat-soluble vitamins, flavours and other components. In summer cows graze on fresh grass, which contains carotenoids and flavour. In winter in most countries cows are fed with silage and/or hay. These are generally lower in carotenoids and flavours. Hence summer-butter is generally more aromatic and more yellow as compared to butter in winter months. In winter butter is also a bit harder, due to a difference in fatty acid composition