Economics is not a word to use to raise jersey steers to beef size in the fall of the year. They should be on free choice GOOD QUALITY hay and the suggestions of 3# 14% feed for starters is good. And to keep increasing so they will keep growing.
Jerseys do good grazing as yearlings and will gain good on GOOD grass then. Until then, they are a dairy animal that needs to be fed like a dairy animal. They have got to have adequate grain or they will get that pot gut look and will actually take longer to feed out.
2-3 % of body weight in grain until they are 8-12 months old... They look to be in the 350-450 wt range in the picture and are in good condition for jerseys. They also do not look "old" so I would say 4-6 months maybe" What is their weight average?
You will be lucky to get them finished out by 18-24 months... Dairy animals put on bone and body before they put on muscle and fat cover. We try to kill ours in the 24-28 month range. We do not push them with grain as much as some do.
Corn silage is a cheaper way to get them to grow and put on body/meat once their bones get grown.
Jerseys make a great beef... I have been eating them for 30 years... but there is no "economical" part of feeding out dairy animal for beef. The best way to do it is for them to stay on a cow until they are 8 months old, getting some "creep grain" along so they know what it is, then on good hay and grain daily. Ours run on pasture and they will weigh in the range of 800-1,000 at 26 months... but we rotate our pastures and they get the best grass we have....