Here are numbers that are helpful when using alfalfa and grass height as an indicator of NDF content:

In general we say 100% grass stands should be cut when nearby alfalfa is 14 inches tall to achieve the desired 50% NDF.

Begin cutting 50/50 mixed alfalfa and grass stands when nearby alfalfa is 22 inches tall for the desired 44% NDF.

Begin cutting 100% alfalfa stands when alfalfa is 28 inches tall for the desired 40% NDF.

The desired NDF for all grass fields is 50%, 50/50 mixed alfalfa and grass fields is 44%, and pure alfalfa stands is 40%. Predicted days to cut are based on daily NDF increases for grasses of 1% point, 50/50 mixed stands of .8% points and alfalfa of .5% point. Typically NDF increases about 0.8 to 1.2/day for grasses expecting the lower end of that range in cooler weather and the higher end in warm. Alfalfa NDF increases about 0.4 to 0.7/day again figuring the lower end of that range with cool temperatures and the higher in warm.  
Growth has certainly been behind this year and that has thrown off the prediction equations to some degree. Alfalfa has appeared stunted compared to grass growth. Shorter alfalfa in alfalfa-grass mixes may mean that grass is making up more of the mix so better to harvest on the early side as you have less high quality alfalfa making up for the higher percentage of lower quality grass. You maybe sacrificing some yield but higher quality should win out.

All the predicted dates to cut have been modified to reflect the faster growth anticipated with the warmer 85-90 F temperatures expected for the coming week. For the most part the grass and mixed stands may only reflect a day or so sooner because the predicted harvest date is so close by. But the warmer temperatures are likely to move up the alfalfa predicted harvest dates up approximately three days.

Hope this information has been timely and valuable to your operation.