The increased use of disc mowers, which are much less susceptible to damaged knives than sicklebar mowers, has resulted in many farmers mowing alfalfa fields considerably closer to the soil surface, less than 3 inches of stubble height.

Decreasing the amount of stubble left in the field will increase yields. Canadian research reported that cutting five different varieties of alfalfa at 2-inch versus 4-inch increased yield in the first cutting by 588 tons per acre and by 172 tons per acre in the second cutting. Research done in Wisconsin also reported that for each cutting of alfalfa the forage yield increased as cutting height was reduced.

However, since the increased yield is due to the harvest of more stem material and not more leaf material, forage quality may decrease. The Canadian research reported a decrease in percent protein as cutting height decreased. The Wisconsin researchers concluded that for each 1-inch reduction in cutting height, forage quality decreased an average of 4 units of relative feed value (RFV). Finally, some forage labs are reporting unusually high ash levels, possibly due to soil contamination from mechanical harvesting, are of concern.

Objectives
To evaluate the current university recommendation to mow alfalfa at a 2-inch cutting height.

Materials and methods
Two studies were conducted to determine if harvesting alfalfa at 2-inch and 4-inch cutting height influences forage quality and yield. The first study involved the hand-harvesting of forage from a second-year stand of alfalfa.

Hand harvest (Prior to field harvest)
Three 24-inch by 36-inch areas were selected randomly within four plots and hand-harvested at 2-inch cutting height for first and second cuttings, while harvest of two areas provided adequate sample for third cut. Sward composition of the field was evaluated by separating forage species, drying and calculating sward composition based on dry weights.

Two inches of stems were removed, weighed, dried and ground for analysis from harvested material. Remainder of harvested plants was weighed, dried and ground for analysis and this material was used to estimate 4-inch cutting height yield and forage quality. The 2-inch cutting height data was determined by mathematically calculating the yield and forage quality from the weighted average of 2-inch stems removed from the plants and the remainder of the plants for alfalfa and grass species. Alfalfa and grass species harvested were sent to a commercial lab for wet chemistry analysis for determination of NDF, ADF, lignin and ash. Chemical and digestibility parameters of alfalfa and grass species were reported individually and as a combined value representative of field composition determined by calculating the weighted average of the alfalfa and grass.

Mower harvest
A limitation of the hand-harvest trial was the lack of foreign material including soil and debris that may be inadvertently harvested when disc mowing. Therefore, a second study involved a replicated field harvest of alfalfa at theoretical cutting heights of 2-inch and 4-inch. Actual cutting height was determined after harvest by measuring stubble height from 15 random locations within each harvest plot. Yield was estimated and chopped forages were dried and ground for chemical analysis.

Conclusions
Overall, there was little or no difference in the quality of forage harvested when an alfalfa/grass mix is cut at 2 or 4 inches. There is an indication of a difference in the yield from the hand-harvest data, with the 2-inch cutting height yielding more forage, as expected. More accurate data on yield from the field harvest of the two cutting heights is needed to determine the milk per acre. It appears that the decision of alfalfa/grass cutting height should be made based on field conditions, needs for forage and grass species planted.  FG

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