Thats correct, you don't have to hay if for whatever reason you can't ie; horsey heath issues. The guideline however doesn't insist that you feed a mega large amount of senior for a daily ration.
One bag of Nutrena Senior, depending on where you buy it, runs approx 15-17 $. By the time you buy a bag of crap feed for $7, buy a bag of beet pulp for $10, flax for whatever amount (it's not that cheap), probiotics, etc etc, you'd been cheaper in the first place to buy the more costly 50# of senior, is my point.
I don't like senior feeds as a sole source of nutrition. Even if the horse has no teeth and has to drink all his feed, I would still give a vitamin/mineral supplement along with beet pulp, alfalfa pellets or other form of process forage, flax seed and rice bran and some Senior thrown in.
Here's what I buy and the costs:
40 lb. Manna Pro Rice Bran: $22.95
40 lb. Beet Pulp Crumbles: $16.99
50 lb. Flax seed (that I grind myself) $32
50 lb. Alfalfa pellets $16.99
That is 180 lb. Of product for $88.93.
Nutrena Senior is about $17 for a 50 lb. Bag.
So 180 lbs. Of Senior would be $61.20.
But you still have to balance the vitamins and minerals that are missing from both plans. So say you buy SmartVite EZ Keeper and give the recommended 50 grams per day, which comes out to about $16 a month.
Recommended feeding of Senior would be 13 lbs. Per day for a 1,000 lb. Horse for Maintenance - no work. This comes out to about $130 a month if feeding Senior. Add the vit/min supplement and it is $146 a month.
I just plugged the following into FeedXL to create a balanced diet for a 1,000 lb. Aged horse not in work:
Nutena Senior 2 lbs.
Ground Flax 1 lb.
Manna Pro Rice Bran 1 lb.
Beet Pulp 5 lbs.
Alfalfa Pellets 6 lbs.
Given the individual costs on the above listed items, plus the 1 ounce of SmartVite supplement, that would come out to $184.28
Even with the Senior and the vit/min supplement, I was unable to get the diet to balance in all categories.
So yes, you could say that feeding ONLY Senior is cheaper but it is also not supplying EVERYTHING the horse needs. And everything would include variety also to keep the horse from getting bored and sour. The Horse recently published a study showing that if horses are given a variety of foods, even if they are stalled, they did not develop the vices and stereotypies that horses do when only fed a single type of feed.
In the wild habitat, horses forage on many different things all day - grasses, weeds, bark, berries, seeds, tree leaves, even brambles and dirt enriched with minerals. It's not natural for them to stand in one spot and eat one type of feed round the clock.
With the variety offered by the more expansive diet, you could soak senior in the morning for a meal, then pellets for lunch and beet pulp for dinner. I do this a lot with my horses and I have never had an upset stomach or diarehea. They'll get flax at one meal then rice bran at the other. I think it's much more of a problem when you're feeding large quantities of sweet feeds or rich grain like corn or oats that you have to worry a lot more about making changes.
Also I buy 4 types of hay from 4 different suppliers and each day my horses get rotated between the types. This is more natural, they're healthier and happier, and always kept interested in what they're eating.
So anyway, yeah you could make the argument that feeding only Senior is cheaper, but cheap isn't always best.