AM or PM?

Mike Rankin

In a recent eHay Weekly, I wrote about this topic and then included an online poll to see where forage producers now landed on this issue after many years of debate. The results: Most just seem to cut without considering the time of day. With over 100 forage producers responding, 62% indicated that they “cut when I can without regard to time of day.” About 23% “prefer to cut only in the late afternoon,” and 15% “prefer to cut only in the morning.”

There was no way to differentiate responses by U.S. region, but that might have been interesting to discern . . . and here’s why.

The preference for late afternoon cutting is actually pretty compelling. Several studies that were done in the western United States showed a clear advantage to cutting hay in the late afternoon or early evening.

The PM-cut forage (both alfalfa and grasses) was higher quality when harvested, and the researchers also had convincing evidence that animals preferred it to AM-cut hay. The result was improved animal production.

Some basic plant physiology concepts drive the preference for PM-cut hay. Plants accumulate carbohydrates in the form of sugars and starches through photosynthesis during the daylight hours. These carbohydrates are produced faster than the plant can get them translocated to the root and crown or use them for growth and maintenance.

Concentrations of these nonstructural carbohydrates, which are nearly 100% digestible, reach a maximum toward late afternoon. In alfalfa, leaf-to-stem ratio also peaks at this time. The arid and semi-arid Western states’ research showed that they were able to hold this quality advantage of PM-cut hay through wilting, harvest, storage, and feeding.

Photosynthesis ceases during hours of darkness, and respiration becomes the dominant plant process. The nonstructural carbohydrates are used during respiration for normal plant maintenance and growth. This leads to a declining concentration in plant tissues until the sun rises again and the cycle repeats itself.

Plants are like people in that they fight to stay alive, even after a traumatic experience like having their legs cut off. Respiration continues until plant moisture reaches about 50%. In fact, photosynthesis continues for those cut plants that are on top of the windrow or swath and exposed to sunshine.

Given the importance of moisture concentration, dry-down rate becomes an essential factor in preserving cut forage quality. Long drying times translate to extended plant respiration following cutting, negating any effect of nonstructural carbohydrate content at the time of cutting.

In the West, low humidity, cool nights, and plenty of sunshine make for fast dry-down rates, especially for the initial dry-down phase to 60% moisture. Hence, cut forage quality is preserved more easily than in the more humid East. In the latter case, cutting earlier in the day is often preferred to ensure forage moisture reaches 50% before the overnight hours, or at least without going more than one overnight.

In regions where rainfall is more frequent than in the arid West, beating the weather trumps any strategy for time-of-day cutting. Morning cutting is almost always going to offer a wider window of confirmed favorable weather, at least in the short run.

There are two ways to optimize cut forage quality at a given plant maturity. One is to mow when it’s at its highest quality, and the other is with rapid dry down. The latter factor is the more important of the two. Though late afternoon cutting may be advantageous in arid Western regions, growers in humid areas will benefit from an earlier cutting that is followed by several dry, sunny days.

Finally, let’s not forget that plant maturity is still king when it comes to forage quality. Overly mature plants, cut at noon or midnight, still make for low-quality forage.