If I remember my livestock nutrition class some 30 years ago certain plants including alfalfa concentrate alkaloids and can become poison just after a hard freeze at the end of the growing season. If your animals are pasturing exclusively on alfalfa at this time it can be a problem.
Looked on the internet, I had to make sure my facts were straight. Alfalfa can be poisonous but not in the way I described. See the quote below.
"Alfalfa (Medicago sativa) is an important forage and silage crop in Canada. However, under some circumstances, alfalfa can cause a variety of different toxic problems. Ingesting rapidly growing alfalfa at the vegetative to mid-bud stage can cause bloat in cattle and sheep (Hall and Majak 1989). Alfalfa has also caused photosensitization in cattle with white skin (MacDonald 1954). Alfalfa contains phytoestrogens which cause infertility in animals, including cattle and sheep. These compounds are also contained in some alfalfa pills that are found in health food stores, and these may cause problems in some cases (Cheeke and Schull 1985). Alfalfa also contains saponins that can interfere with the growth of poultry and thus reduce egg-laying (Fuller and McClintock 1986, Oakenfull and Sidhu 1989). Low saponin cultivars have been developed."
I guess the old saying that to much of a good thing can be bad, applies to alfalfa.