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He and New Zealand team-mate Ian Woolley, of Blenheim, competed against representatives from more than 30 countries at Baden-Wurttemberg, about 30km south of Stuttgart, on Sunday and Monday (New Zealand time).
Mr Mehrtens came third in the reversible grassland section, 15th in the stubble, and eighth overall in the reversible ploughing events.
Mr Woolley was third in the conventional grassland section, fourth in the stubble, and third overall in the conventional ploughing competition.
The Kiwis arrived Stuttgart on August 8, relieved to find their tractors had also arrived intact.
They were put in touch with a farmer who had travelled through New Zealand 15 years ago and was happy to help them settle in.
The ground was hard and dry when they first tried ploughing it.
An email from New Zealand team manager Alan Wills on August 26 said the ploughs were going well but some teams were having trouble with the dry conditions.
''The site is going to be superb for a world ploughing contest.
''To access it you drive up through a typical area of Black Forest and then on to a plateau of several hundred hectares.''
On August 29, Mr Wills emailed an update after two full days of practice.
''We chose to start in the grassland, which is hard and very dry.
''In a short time the plough inspection team had visited and had failed Ian's plough on account of a plough wheel attached to the right-hand hydraulic linkage arm was contrary to the rule book and was not to be used and should be removed.
''We did challenge them, so they deferred the issue to the head steward's team. We were advised that night that the decision stood.''
After further discussions, the New Zealand team had to accept the ruling and ruled Mr Woolley could not use the wheel during the competition.
Mr Wills said he would write to the World Ploughing Organisation after the event to explain three people with international experience were involved with Mr Woolley's set-up.
''There are many teams who are struggling in the grass and I am expecting there to be a significant tail in the points,'' Mr Wills said.
''Apart from a few plots in the grassland competition, both practice and competition plots are to be ploughed across the slope. This is certainly challenging most conventional ploughmen, particularly with the lower side of the crowns.''
Mr Woolley had more problems, as on two occasions the same coulter stalk broke where it had been welded after hitting rocks in the stubble bank.
A trip to an engineering shop was needed.