The key management strategy in producing high quality alfalfa forage for lactating dairy cows is to select the optimum harvest date for the first cutting. Second and successive cuttings should then be harvested every 28 to 30 days thereafter. A common approach to determining the optimum harvest date for quality and quantity is by observing the growth stage of the plants. There is a direct correlation between the morphological development of the plant and the quality and quantity of alfalfa harvested. It is well documented that alfalfa nutrient quality decreases as the plant matures from the bud stage to full flower. At the same time, pounds of plant material harvested per acre increases as the plant matures. The optimum harvest date is a compromise between the feed quality and the tonnage produced. The growing degree days concept has been used to predict alfalfa growth in various studies, throughout the United States. Growing degree days (GDD) may be used as a forage harvesting management tool for alfalfa producers.

MATERIAL AND METHODS

A study was conducted in three locations in southwestern Illinois to help establish a useful GDD guide to selecting the proper harvest time for first cut alfalfa. The data collected consisted of monitoring alfalfa nutrient content and plant height twice a week on three different fields from April 25 to May 19, 1994. Data was also compiled on the accumulated GDD (base = 50EF.) supplied by Alvey Agriculture Research, Carlyle, Illinois. Dry matter tonnage estimates were made based on harvesting an 18 square foot area once a week during the 4 week study. Field A, variety WL322, was an established alfalfa crop in the third year of production. Field B, variety NK-Fortress, was also a third year alfalfa stand. Field C, variety WL322, was a first year stand seeded on August 20, 1993. All three fields were fertilized based on soil test and plant material removed. Alfalfa samples were tested using near infrared reflectance spectroscopy (NIR) at Alvey Laboratory Inc., Belleville, Illinois.

RESULTS

There was consistency among all three fields in decreased nutrient quality and increased tonnage as growing degree days (GDD) accumulated. Based on the 1994 data, May 12 was the optimum harvest date to achieve an alfalfa relative feed value over 150. Once the growing degree days averaged above 300, (RFV) forage fell below 150 RFV.

Field A - Effect of maturity on alfalfa quality and quantity

Field B - Effect of maturity on alfalfa quality and quantity

Field C - Effect of maturity on alfalfa quality and quantity

CONCLUSION

With limited data from one year, it is difficult to conclude that growing degree days is the sole indicator needed to select the optimum alfalfa harvest date. However, GDD can be a useful management tool, along with visual observation of plant growth, to establish a desired range for when first cutting should begin and be completed. Alfalfa cuttings that will be used for high producing dairy cows should be harvested between 250 and 275 GDD. Based on growing conditions in southwestern Illinois and the results of this study, all alfalfa harvest should be completed at 375 GDD. Realizing that the earlier cut alfalfa will be of higher quality, producers should inventory forages and feed according to the nutrient needs of the various livestock. A continuation of this study will be conducted to further test the data and review other factors that may influence the GDD and its relation to harvest date. Funds to conduct this field study were provided by the Illinois Cooperative Extension Service 1994 Competitive Block Grant.