What machinery was developed to improve grain harvest?

Could be several of them! You have cultivators, which are kind of a development of earlier ploughs, seed drills, which allowed the planting of many more seeds than scattering them by hand (and stopped the birds from eating them) or combine harvesters, which were much more efficient at gathering grain in than the old method of harvesting by hand with a scythe. Depends a bit on what period you're talking about. I would have said the seed drill is probably the most likely answer as it certainly did the most to improve grain harvests by allowing corn to be planted more thickly and reliably.