In theory, you are correct in that at night when photosynthesis ceases, the plant sugars tend to go to the roots. Obviously this is not an instantaneous response but continues throughout the dark cycle. What this means is that the sugar content of the plants will be lowest in the morning, early daylight hours, and really the best time to graze would be beginning at daylight when the root levels of sugars are highest and photosynthesis starts up again. Having said all this, the difference in the total amounts of sugar during the peak of photosynthesis and the nadir, lowest level or no photosynthesis, is not very great. In other words, the difference in the total calorie content is very minor, and you are probably, as one forage specialist says, "splitting hairs." The total sugar content of a forage plant and the effect on calorie content during the day and night are minimal.

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