A crop cutting apparatus comprising: a frame structure arranged for forward travel over ground having a standing crop thereon; a cutter bar secured to the frame structure and extending transversely across a front end of said frame structure; a plurality of knife guards mounted in spaced relation along said cutter bar and projecting forwardly therefrom; each of said guards having a ledger surface with opposed side edges (15A, 15B) thereof arranged to provide first and second shearing edges; a first and a second sickle bar (24A, 24B) mounted in transversely extending position and being driven for reciprocating movement relative to said knife guards; the first and second sickle bars each having a plurality of knife blades mounted thereon for movement therewith with each of the knife blades having two side cutting edges (23F, 23G) which cooperate with said shearing edges of said knife guards; a drive system (36, 37) driving the first sickle bar through repeated cycles of reciprocating movement and driving the second sickle bar through repeated cycles of reciprocating movement; wherein the drive system is arranged such that the reciprocating movements of the first and second sickle bars occur at a phase angle difference; the drive system having a phase difference control component (601, 605) operable to drive the first and second sickle bars at a first phase angle difference and at a second phase angle difference which is different from the first phase angle difference, wherein the crop cutting apparatus according to any preceding claim wherein the second phase angle difference is variable in response to changes in ground speed or in response to a sensed load applied to the drive system (36, 37) by the first and second sickle bars (24A, 24B), characterized by the drive system comprising a hydraulic circuit including hydraulic motors (38, 39) supplied by one of more pumps and the sensed load to the drive system is detected by changes in the hydraulic circuit. a frame structure arranged for forward travel over ground having a standing crop thereon; a cutter bar secured to the frame structure and extending transversely across a front end of said frame structure; a plurality of knife guards mounted in spaced relation along said cutter bar and projecting forwardly therefrom; each of said guards having a ledger surface with opposed side edges (15A, 15B) thereof arranged to provide first and second shearing edges; a first and a second sickle bar (24A, 24B) mounted in transversely extending position and being driven for reciprocating movement relative to said knife guards; the first and second sickle bars each having a plurality of knife blades mounted thereon for movement therewith with each of the knife blades having two side cutting edges (23F, 23G) which cooperate with said shearing edges of said knife guards; a drive system (36, 37) driving the first sickle bar through repeated cycles of reciprocating movement and driving the second sickle bar through repeated cycles of reciprocating movement; wherein the drive system is arranged such that the reciprocating movements of the first and second sickle bars occur at a phase angle difference; the drive system having a phase difference control component (601, 605) operable to drive the first and second sickle bars at a first phase angle difference and at a second phase angle difference which is different from the first phase angle difference, wherein the crop cutting apparatus according to any preceding claim wherein the second phase angle difference is variable in response to changes in ground speed or in response to a sensed load applied to the drive system (36, 37) by the first and second sickle bars (24A, 24B), characterized by the drive system comprising a hydraulic circuit including hydraulic motors (38, 39) supplied by one of more pumps and the sensed load to the drive system is detected by changes in the hydraulic circuit.