Nah, no worries, Dennis. Doesn't Steer Manure sound more "manly"? Like it is good strong stuff that will be much better than any old "cow" manure. Good marketing ploy! LOL!
In actuality, cow manure is normally, 11-7-10 for its NPK. Steer manure is normally 14-5-8 for its NPK and therefore has a little bit more nitrogen in it and less of your phosphorus and potassium. So check the NPK levels to see what they are (should be listed on the package).
Your N will produce your green foliage. Too much and you get lots of leaves but not as much flower production. Too little and you get yellow leaves and not a lot of new growth.
P gives you strong roots and flower production. Too little and you get dull foliage, weak stems, and buds that refuse to open.
K gives you vigorous growth. Too little and you get weak stems, poorly developed buds, leave edges that turn yellow and then brown.
I would go ahead and use the steer manure but alternate it with the fish emulsion. Usually the fish emulsion has other nutrients in it that roses need. If your plantings are new, I would stick with the fish emulsion for now until they mature a bit. You don't want to over feed or burn any roots. Remember, a little goes a long way. Especially if your roses are in pots. You may just need to feed them on a more frequent basis than those in the ground. Check your leaves, buds, etc., for signs for what they may need.
For me, I find it easy to over feed my plants (that old saying that if a little is good, then a lot must be better---so not true!). Your conditions are so different than mine. I live where it is much hotter and dryer, so therefore, I tend not to fertilize at all in the summer. My plants are just trying to survive. Even the ones in pots. They get fed about 3 times a year, if they are lucky--with a good slow release organic fertilizer and supplements (if they are lucky) . However, I am going to try an experiment this year, to see if one additional feeding for the ones in pots improves them or stresses them out too much (yes, my garden is a continual experiment for me). Of course, it depends on the weather. We have been 108 F this week so it is way too hot to feed any of my plants. Mostly, they will be getting an additional application of compost.
I think you told me that you live on the dryer side of the island, so I am not sure how much rain you get annually (I am lucky to get 10--11 inches in a year and this year it was less). So I don't know how much rain or water you have that might leach out the minerals, etc., from your pots. So that will make a difference too. Gosh, I hope I didn't confuse you. It is about 3 am here and I need to get back to bed and sleep. So give it a whirl. Don't over fertilize. Wait until your new plants are a bit more mature before you try the steer manure on them. A little does go a long way. See how your plants respond to what you do and use. I have some roses that are heavy feeders and some that are not. As one of my teachers told us, answer questions with, "It all depends...".