Why would Joey and Zoey be relieved that Albert had taken over the horse work from the farm ?

They were relieved because they were able to spend more time with Albert and less with Albert's father.

The war hardly touched us on the farm to start with. With more straw still to turn and stack for the winter, old Zoey and I were led out every morning early into the fields to work. To our great relief, Albert had now taken over most of the horse work on the farm, leaving his father to see to the pigs and the bullocks, to check the sheep, and to mend fences and dig the ditches around the farm, so that we scarcely saw him for more than a few minutes each day.