It's true. Sugar levels peak in late afternoon going into EARLY evening. Peak photosynthesis for the day has taken place and is tapering off as the sun gets lower in the sky and the plant has stored as much sugar as possible for metabolism overnight. Unlike us, plants can't get up for a midnight snack! So, they 'eat a big supper' to hold them to breakfast, so to speak. And, like our blood sugar levels, plant sugar levels are lowest in the early morning before breakfast. Additionally, the plant's stomates are usually closed in noon- early afternoon time frame because of the heat of the day. The stomates open in early morning to release oxygen and take in carbon dioxide (releasing water vapor is a usually undesired byproduct of the stomates being open) and then close as the heat of the day comes on to minimize water loss. Then they open again later in the afternoon to release oxygen and absorb more carbon dioxide when the evapotranspiration rates are lower because the sun is lower in the sky. Cutting in early morning or late evening usually allows the hay to dry faster because the plant opens the stomates for gas exchange and when it is cut, the stomates stay open for awhile, and that undesired moisture loss from the stomates speeds the wiltdown of the plant. Cutting in early morning though, results in less sugar in the plant, than higher sugar levels for the overnight plant metabolism that is present in early evening.
I've read of some studies that show that well fed cattle, given a choice between hay cut in the morning to noontime period or hay cut in the late afternoon- early evening period will choose late afternoon or evening cut hay nearly every time, assuming the hay is of equal quality. Unlike the old butter/margarine commercials, cows can tell the difference.
Now, that being said, the old axiom "make hay while the sun shines" is good advice, if conditions are unfavorable or risk becoming unfavorable as they usually do. I guess it depends on the situation. If I were growing hay in a dry climate with little chance of inclement weather interfering with hay quality, I would probably shoot to cut in the late afternoon-early evening time frame. But if weather is an issue, it's cut it whenever you can and hope you have enough time to get it dried and rolled or in the barn before it rains you out again. Basically, if everything is perfect and you want to up the sugar content, shoot for late afternoon cutting. If the grass is over mature and quality is pretty shot anyway, and the weather is finally cooperating, I might try to cut in the afternoon to at LEAST get the sugar content up, even if the nutrition and digestibility are already shot. BUT it certainly isn't worth risking rain damage or losing the cut, or losing more digestibility or nutrients just to get a couple more points in the sugar category, either. So, as with most things, it depends on your situation at the time...
Good luck! OL JR
