Heartrate monitors are useful tools for athletes like rowers, cyclists and distance runners. I think they have little application to rugby training.
Consider the requirements of your position and plan accordingly. Every player will benefit from working on their speed. There have been a few threads on the subject in the past. If you post some more details about yourself I'll happily offer some advice.
As an aside, I recently saw a promo for a crosstraining event - row 500m, do some step ups, lat pull downs, bench, run 1km etc. The press (something like 20kg for 40) was illustrated, I kid you not, by a picture of a guy stripped to the waist, seated, wearing a HRM and gloves.
Oddly, the legend 'Magnificent Bell-end' had been omitted.
What was he going to do? Overshoot his target heart rate and drop out of the competition?