It is well known that alfalfa is accumulating dry matter at a maximum rate in the bud stage which only begins to drop off at 1st flower. This is why alfalfa at 1st flower or 10% bloom will produce the most nutrients per acre.

Each inch of lower cut stubble will yield something between 0.1 and 0.15 ton per cut per acre. Under good growing conditions, according to recent research, dry matter accumulation is over 200 pounds per acre per day at first and second harvests. (On a 35 day harvest schedule any farmer knows there is very little growth for the first two week. It will then therefore take 200 pounds per day to reach a 2 ton cutting,) The lower stubble without leaves will run about 10% protein and have a RFV (Relative Feed Value) below 100. For dairy quality hay you may choose to cut an inch higher and a day later. Here's why.

If you are putting up hay, a higher stubble will speed curing. Even if you are putting up haylage, a higher stubble cut may be kinder to the alfalfa, even according to those recommending the low cutting heights. In addition, later harvesting aids in persistence and faster recover after harvest which usually means better yields over the life of the stand.

Consider the advantage of higher quality varieties. Alfalfa drops at a rate of about 2 to 5 points per day at the bud stage. Since some high quality varieties run 10 to 15 RFV points above conventional varieties, this means you could wait another 2 to 5 days and still have the same quality. By harvesting higher on the yield curve, you will maximize yield and persistence. Remember that persistence has two implication, it may mean the stand may winter-kill, or that stand is thinning and yields declining. (RFV drops the fastest in the first cut and/or high temperatures)

Some universities and popular press have picked up on some research that each inch higher cut will result in a 0.5 ton yield loss per acre per inch. This doesn't seem possible since average northern U.S. alfalfa yield is between 4 and 5 ton. At 0.5 tons per inch, 4 tons would be had with alfalfa 8 inches tall. Admittedly yield will drop off as you move up the stem, still............. Some other research shows every inch yields closer to 0.25 ton. The 0.1 to 0.15 ton per cut above is generous and would amount to 0.3 to 0.45 ton per year in a 3 cut harvest system.)

Advice on variety selection:
If you are having problems making dairy quality hay, use high quality varieties under the harvest schedule you are presently using.

If you are having problems with curing, stand thinning or persistence, higher quality varieties can be helpful.

If you are putting up more hay than you can get to over a 3 to 4 day period, higher quality varieties for later harvest can be useful.

If you are have none of the above problems and have no problem producing dairy quality hay or are harvesting beef cow quality hay at 1st flower or 10% bloom, look only for the highest yielding variety.

Advice on cutting height:
For dairy quality hay cut low enough so as not to leave any leafy material in the field. For non-dairy quality hay, cut as low as practical.
nwroc.umn.edu/Cropping_Issues/2005/issue3/06_07_05_no_6.htm
http://www.ag.ndsu.nodak.edu/plantsci/forage/alfalfa.htm