My hay yields are not as good as they used to be. What is hurting the yields? Cutting machinery, variety, fertility,....?

SHORT ANSWER: It is unlikely that harvest management practices are the cause of this reduced alfalfa hay yield. Most likely, low soil fertility is the cause of the problem. Early symptoms of low soil fertility are low yields and slow regrowth.  

EXPLANATION: Cutting frequency, height, date, etc., can vary widely and seldom cause drastically reduced alfalfa hay yields. Poor decisions having to do with variety, insecticide, herbicide, and site selection usually cause stands to thin quickly.  

After a few years stands thin more quickly under low soil fertility. It is likely this stand has not yet reached that point, but will soon. Unfortunately, most producers first suspect factors other than low soil fertility or low soil pH.  

Good alfalfa production for several decades removes large amounts of fertilizer elements from the soil. Many good fields in Oklahoma were highly fertile for several decades after the prairie was first plowed, but soil fertility has slowly declined. With a relatively small decrease in fertility each year, it is difficult to notice that the soil can supply only a fraction of the amount of phosphorous and/or potassium necessary for good alfalfa production.  

Current soil test calibrations predict about 10% yield response when a deficiency of 60 lb/A P₂O₅ is alleviated. For a good field of alfalfa that represents about 1/2 ton/A increased yield. Conservatively, this is $40/A income increase from fertilizer that cost $12 to $15/A to purchase and apply -- cost/benefit ratio of about 1:3.  

Each alfalfa field should be soil tested annually. Soil testing is the only way to know if (or how much) fertilizer should be applied. It is a good idea (and inexpensive) to check the fertility level every year, even when no obvious problem exists and twice a year when there is a problem. Once the fertility has become limiting, large amounts of fertilizer must be applied every year to maintain high yields.  

RELATED COMMENTS: Low soil pH can be corrected with a single application of agricultural lime before establishing the field; however, it may be necessary to reapply lime after 6 to 8 years of production. Lime applied after establishment will take longer to correct the low pH problem.  

Soils "severely" deficient in phosphorous or potassium, on the other hand, are more difficult to improve quickly. Even when recommended amounts of these materials are applied before establishment, it will be necessary to apply phosphorous and potassium each year.  

A certain amount of phosphorus applied as fertilizer is absorbed by the soil and is not available for plants. In addition, large amounts of phosphorus and potassium are removed each year in hay. Five tons of alfalfa hay contain approximately 70 lb P₂O₅ and 300 lb K₂O. This amount is removed as part of the hay crop and must be replaced by fertilizer or soil weathering. Soil weathering is a complex process whereby chemically bond minerals are released by the soil. Regular soil testing is the best way of accounting for how well soil weathering restores crop removal of phosphorous and potassium.  

Low fertility and pH have far-reaching effects on alfalfa. In addition to reducing yield and shortening stand life, as noted above, they can exaggerate the effects of pests. Low fertility conditions result in weak alfalfa plants that do not compete well with weeds. Plant stress caused by low fertility coupled with insect infestations can result in losing an alfalfa stand in a short time. In contrast, well-fertilized alfalfa plants without insects, compete with broadleaf and grassy weeds very well, resulting in very little weed encroachment for several years.