Peter Vallis's team of mules pull an I&J tedder at Horse Progress days near Lancaster, Pennsylvania. The tedder fluffs up the newly mown hay so that it will cure more evenly and quickly. I&J is an Amish firm that makes implements for horse and tractor farmers. Mules are the offspring of a mammoth jack donkey and a mare horse. Mules combine the best features of both species but are almost always sterile.