For maintenance horses need from 1.5% to 3% of their body weight in good quality forage a day. This means 15-30 lbs for the "average 1000 lb horse" . Easy keepers may do fine on the low end and hard keepers may need even more than the 3%. And you need to know what this cutting is, not the previous because the protein content changes as the year progresses. And if it's brome grass I would double check the original assessment as it tends to be about 11% protein at it's best as hay---about 18% protein when growing.
When you add in work you need to add in more digestible energy to meet the additional requirements. I would choose a feed that is designed to meet the needs of a working horse like Strategy when you increase the workload again or you could just increase the amount of Safe Choice according to the label.
Licensed Veterinary Technician