Re: TDX tracking training..
I started TDX work right along with TD work. My dogs know how to negotiate cover changes and obstacles within a few months of learning their first turn. Since I always use multiple articles having more articles is never a problem.
I would go back and forth on distance, a short fun track and then a 600 yard track, then short then 650 yards and so on. You also have to factor in increasing age. Many people have this formula for increasing age by 15 minute increments. If the dog can track a 2 hour track he can track a 3 hour track. But even with age I do fresher tracks interspersed with older tracks.
Most dogs have problems with the distance not age. Some dogs will take the second cross track because they are tired and fresher scent may be easier for them. I train cross tracks very rarely. When I do, I put an article about 10 paces past the cross track to pull the dog over and offer an opportunity for reward right away. Then I increase the distance between the cross track and article until the dog does not need to be pulled.
As for hard surfaces- start soon. I use baby steps and a lot of articles to pull the dog out on the hard surface. They know to find articles and indicate so the articles help them learn the track can go other places than grass too. I also use a squirt bottle to spray water ahead of me on the asphalt and then I walk through it. That helps keep the scent on the ground better during the learning phase. You will want to fade that fairly quickly by using less frequent squirts of water. After you fade the water start taking a longer step until you can use a full lenght stride. Space the articles further away until uoi can cross a parking lot with one artcle on it.
When you introduce the MOT (Moment of Truth turn in the open on hard surfaces) use shorter strides into and out of the corner. I use chalk to mark it so I can see exactly what the dog does at the point the turn was made.
Same tracking principles apply here, you want the dog to work as closely to the track as possible, but understand the scent will drift more on the hard surface, kind of like a puddle of water.
A good book on VST is available on http://www.pawmark.com
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