Nutritional value of forage after a frost
In fall, as plants mature, they lose overall nutritional value, but at a slower rate than during the summer due to cooler temperatures and shorter days.
During the fall season, grasses gradually accumulate nonstructural carbohydrates (NCS) aboveground, especially in the basal part (lower stems/leaf bases) of the plant. NSC accumulation is a gradual process and is usually higher in more mature forage.
Quality of both legumes (i.e. alfalfa) and grasses begin to decline after a hard "killing" frost. Legume quality deteriorates more rapidly than grass quality because legumes will lose their leaves and grasses do not (leaves contribute significantly to the overall quality of both grasses and legumes).
Grasses often become more palatable (preferred) because of the elevated NSC values. As discussed in the "Ask the Expert" article, legumes (i.e. alfalfa) and grasses tend to have elevated NSC values (an indicator of starch and sugar levels) after a frost, and it is recommend that horse owners wait up to a week before resuming grazing after a killing frost in an effort to avoid some health problems (NSC values will eventually decrease over time). Also, forage protein, equine total digestible nutrients (TDN), and equine digestible energy decrease gradually after a hard frost. This decline is due to a combination of the forage plants leaching nutrients and continued plant respiration.
Bottom line: grass species tend to retain their nutritional value longer after a killing frost (compared to legumes), and horse owner should wait up to a week before resuming grazing after a killing frost.