This is a topical question as some grasses now produce horrible barbed seeds that only migrate in one direction in the ear, or penetrate feet and skin.
Protecting ears alone will not save the dogs, as more harm often comes from seeds that have gone in through other parts of the body.
It has been suggested that shaving all the hair from the insides of the ears to stop seeds sticking may be a cure, but I know for a fact this does not work.
The only answer is to avoid training or exercising dogs in areas where these grasses are found in the summer months.
Inconvenient though it is, there is really no other answer.