The onset of the Great War had a profound effect on the history of the tractor. British farmers had been wary of the initial capital outlay required to buy one and were concerned about keeping flammable material about the farm. But with farm workers being called up for military service leaving farms undermanned the British government ordered 7,000 Fordson tractors from the US to try to alleviate the problem. The German submarine menace reduced imports to a trickle and the subsequent need to produce much greater quantities of food forced UK manufacturers to start developing tractors themselves. By 1917 tractor design was beginning to form its own line with machines becoming much lighter and less likely to compact the soil. The ‘Ideal’ and the three-wheel Austin Culti-tractor, a 20 hp four-cylinder machine of 35cwt were two examples. In September 1919 the Society of Motor Manufacturers and Traders and the NFU jointly ran their first trial and exhibition at S Carlton, Lincs and the following year a very high number of exhibitors from both sides of the Atlantic participated. The Austin Model R was produced from 1919 and used an Austin four-cylinder petrol/paraffin engine. This had a bore and stroke of 3.75 x 5 in and developed 27.39 hp at 1500 rpm. The tractor had two forward speeds and one reverse with a cone-type clutch and was fitted with a tubular radiator and very large belt pulley. A Zenith carburettor and North and Sons (Watford) Ltd magneto were used.