Non Structural Carbohydrate Concentrations During Wilting of PM- and AM-Cut Alfalfa.

Increased concentrations of non structural carbohydrates (NSC) in forages improve N use efficiency in dairy cows. We measured the changes in NSC concentration during wilting of PM- and AM-cut alfalfa. In 2008, field-grown (46°48′ N; 71°23′ W) alfalfa (cv. AC Caribou) was mown into wide swaths at the early flowering stage of development either at 18h30 (PM) of a sunny day or at 8h30 the following morning (AM). Triplicate forage samples were taken at cutting times and every 2 hours from 8h30 until the end of the second day. This 2-day experiment was conducted twice in spring and summer, and once in fall (n = 223). Concentration of NSC (glucose + fructose + sucrose + pinitol + starch) was chemically measured in a calibration set of samples and then predicted by near infrared reflectance spectroscopy.

Concentration of NSC (mg/g DM) in PM- and AM-cut alfalfa at times of cutting* and every 2 hours following the AM-cutting.
Sampling			Spring growth			Summer regrowth			Fall regrowth
Day	Time		PM	AM		PM	AM		PM	AM
1	18h30		85.7*			106.3*			134.4*
2	8h30		87.4	73.4*		99.2	89.8*		128.7	110.4*
2	10h30		81.0	77.5		101.7	80.4		131.8	110.2
2	12h30		72.6	58.7		101.0	84.0		129.2	116.7
2	14h30		73.5	68.5		95.5	87.4		129.1	111.6
2	16h30		69.8	72.4		96.0	86.3		133.9	110.9
2	18h30		69.2	64.7		90.0	73.5		126.4	107.5
2	20h30		65.9	67.5		97.6	81.9		117.2	102.5

At cutting, NSC concentration was greater (+ 17% in spring, + 18% in summer, and + 22% in fall) in PM- than in AM-cut alfalfa. Concentration of NSC remained greater in PM- compared to AM-cut alfalfa throughout the wilting period for the summer and fall regrowth but not in spring growth; wilting conditions were poor in spring. Cutting alfalfa in PM improves forage quality.