Early combines, some of them quite large, were drawn by horse or mule teams and used a bull wheel to provide power. In 1902, a combine could harvest enough grain in one hour to make 10 loaves of bread. Tractor-drawn, PTO-powered combines were used for a time. These combines used a shaker to separate the grain from the chaff and straw-walkers (grates with small teeth on an eccentric shaft) to eject the straw while retaining the grain. Tractor drawn combines evolved to have separate gas or diesel engines to power the grain separation. Today’s combines are self-propelled and use diesel engines for power. A significant advance in the design of combines was the rotary design. Straw and grain were separated by use of a powerful fan. Rotary combines were introduced by Case IH in the late 1970s. About this time on-board electronics were introduced to measure threshing efficiency. This new instrumentation allowed operators to get better grain yields by optimizing ground speed and other operating parameters.