Several things help bring about fertile breeding cycles and then there are other things that are just indicators of breeding readiness.
First, are they showing strong heat now?
What sizes are their actual follicles?
How long have they been showing heat?
Are they being teased vigorously by a stallion? Teasing stimulates heat cycles and is necessary in many mares. We have found that daily 'aggressive' teasing makes a HUGE difference in some mares.
Are they eating good, green grass? Adequate intake of Vitamin A is VERY necessary to have fertile heats. Supplementing Vitamin A is very beneficial, even for mares on grass. We use the injectable form only give it to them orally. A good supplement that is available world -wide is Farnam's 'Mare Plus' (so named because of its high Vitamin A levels). It takes 30 to 60 days to actually show an improvement which is why we use the oral doses of the injectable Vitamin A-D.
Are they in good flesh? Not too thin and not too fat?
Are they 'slick' and completely shed off? You have longer heat cycles before a mare is completely shed off or at least shed slick on the neck and shoulders? This is a very good indicator of readiness to have a fertile heat.
Transitional heats are usually earlier in the season than where you are now in your summer. You are only 1 month from your summer solstice. you should be seeing shorter heats by now.
Transitional heats, while longer, are usually fertile. You just have to faithfully follow them since the mare will ovulate a day or so from the end of the heat cycle. That can obviously mean a lot of teasing, palpating and/or ultrasounding.