Fall Management of Alfalfa
By Stephen K. Barnhart, Extension Forage Specialist, Iowa State Univ., Anes, IA
19 Sep 2008 -
Fall Management of alfalfa
Steve Barnhart, Extension Forage Agronomist, Iowa State Univ. Ames, IA
A rainy May and early June in 2008 put most alfalfa producers behind 2 to 3 weeks for their 1st and correspondingly their 2nd and /or 3rd cuttings. Now we are in mid-September, and producers are taking advantage of a favorable week of drying weather and making what is their ‘last summer cutting’. This is crowding into the ‘normal’ 4 to 6 week ‘fall rest period’ for the alfalfa stands. Have (or will) these mid-September harvests put the stands at risk ?
Our locally adapted alfalfa varieties (and most other perennials) are genetically ‘programmed’ to respond to external factors. Their response to shortening days and cooling average daily temperatures of fall is to begin the gradual ‘cold hardening’ process. ‘Cold hardening’ will continue for 6 to 10 weeks. The genetics of the variety determines how cold tolerant the plant crown and taproot can be during the winter months. Most winterhardened alfalfa plants can withstand soil temperatures in the crown area to about 0 to 4 F without crown tissue damage. At lower soil and crown temperatures, varieties and individual plants will vary in the degree of cold damage they may experience.
Fall Management strategies and considerations for alfalfa fields for the remainder of the fall season:
Q. Will you save this field for hay next year?
No. harvest anytime. (Only slight reduction in N credits to next crop if last top growth is removed.)
Yes. Take last summer cut by late August or very early September, and leave fall regrowth stand in the field (no late cut or grazing !!!) If you have not fertilized
in the summer topdress any needed K in late Aug or Sept.
Q. My alfalfa is knee high in mid-Sept, should I cut it then?
Will the field be hay next year?
No. Cut anytime.
Yes. Do you NEED the hay?
No. Leave the last growth in the field – don’t graze in fall or winter
Yes. It is best to wait until at or after the killing freeze (23-24 F or colder) to cut, and leave a 4-5 inch stubble.
Q. But I can’t dry hay in October! (Not very rapidly !) What is the risk of cutting in mid-
Sept.?
The best scenario is that the alfalfa plants have accumulated ‘root reserves’,
and, if left uncut, can do very well in the coming winter.
The next best scenario: If you cut in mid-Sept. the plant will begin to regrow and begin to use the stored ‘reserves’. During fall regrowth, root reserves will decrease for a week to 10 days while new growth is starting, and will gradually rebuild ‘root reserves’ until a killing freeze (23-24F or colder for several hours) stops seasonal growth. If the ‘fall rest’ or fall regrowth period is long enough (4 to 5 weeks), the plants will likely recovery sufficient ‘root reserves’ for good plant vigor and winter survival.
The worst scenario: During fall regrowth following a mid-September cut, root reserves will decrease for a week to 10 days while new growth is starting, and will gradually rebuild ‘root reserves’ until a killing freeze (23-24F or colder for several hours) stops seasonal growth. If the ‘fall rest’ or fall regrowth period is not long enough (less than about 4 weeks), the alfalfa plants will be left with a relatively low level of available ‘root reserves’ and will have minimal levels of ‘reserves’ to both get through the winter and to regrow from in the spring. From a low level of stored carbohydrates, even a minor premature spring recovery and freeze-back will put the plants in a very poor physiological state.
Factors which improve alfalfa winter survival:
- Management of insects (potato leafhoppers) during the growing season
- Good levels of available potassium in the soil
- A variety with winter/cold tolerance
- A variety with a good ‘disease resistance’ traits
- 3 summer cut harvest systems with good regrowth between cuttings
- 5-6 weeks of uninterrupted growth during September and October
- All of the last growth of the season left in the field (no cutting or grazing)
- Soils with ‘average’ soil moisture or slightly drier during fall and winter
- 4” or more of winter-long snow cover
- Young stands (1st or 2nd production year) – we see more consistent winter
injury in older stands (3rd production year and older)
