A good deal of the success with hotbeds is due to the careful selection of the manure. Cold manure, like that of cows and pigs, should never be used because it will not heat. Horse manure is best, but in this case at least one-third of the bulk should be straw. If pure manure is used, it will pack too tightly when firmed, so that it will not heat. If possible, the manure from grain-fed, straw-bedded horses should be used.