Many of the grazing gurus of the planned grazing methods make a big issue over needing cattle to graze in a non-selective manner. I have to admit, I've done my share to add to the confusion, so now it is time to (try) and clear up the muddy waters.
Much of our feeling that selective grazing is bad is due to the ultra-selectivity used by cattle in a set stock situation. They get so selective that they go back and re-graze individual plants they grazed a couple of weeks earlier, so that those plants never get a chance to fully recover, while they completely ignore plants of the same variety only inches away. As a result, fewer plants are able to reach some semblance of a full recovery.
The cycle starts again the following growing season with those plants which "fully recovered" greening up first, followed by the surviving overgrazed plants, with those which were not grazed greening up last. These plants which were not grazed the previous year will very likely go un-grazed again and begin to oxidize and slowly die, a blade or two at a time. The cycle repeats with plants being either over grazed or under grazed until weeds start appearing as the grass dies off.
While this overly selective grazing is a bad thing, a certain amount of selective grazing is actually necessary. Different grasses and browse plants mature and are ready to be grazed at different times. In our efforts to mimic the wild herds we have forgotten this aspect, as well as the fact that the large wild herds have always come together and fallen apart depending on the time of year, and abundance of feed and water. While high animal density and non-selective grazing is beneficial while grazing irrigated pasture, under range conditions we need to know when to use it, and have the flexibility to to adjust for those times when, nutritionally, the cattle to be highly concentrated or slightly spread out to take advantage of the higher quality forage which may not be highly concentrated, especially on desertified, degenerated rangeland.
A good example would be a pasture which has a large stand of tabosa or alkali sacaton in part of it while other parts of the pasture is made up of a variety of sparse grasses and browse. I order to get optimal nutritional benefit of the tabosa and alkali sacaton, the cattle will need to be highly concentrated. In conditions where there is a large mono culture (or a wide diversity of plants ready to be grazed) we are looking for, and need them to practice non-selective grazing.
In these conditions, if their herd instinct has been rebooted, they will close together in strips. Unlike when they are being forced into non-selective grazing with fences, the cattle will not graze to the ground, but leave half to two thirds of the plant. Leaving half to two thirds of the plants allows for faster recovery of the plants, and overall, more animal grazing days per acre, and a higher average daily gain on cattle.
Conversely, when the cattle are grazing a minimal pasture it is highly unlikely that one will be able to accurately judge the amount of feed the cattle will receive. There is a 90%+ probability that cattle being forced into non-selective grazing under these conditions will lose weight instead of gain, even if protein supplements are added. Rebooting herd instinct and placing cattle to migrate these marginal areas will allow them to select enough of the plants they need to maintain condition while making enough impact to begin regenerating the soil and grass. Even if feed density requires that the cattle migrate through fifty or even a hundred feet apart, it is still more than enough impact to stimulate the microorganisms in the soil and new forage growth.
In nature, the "large herds of herbivores" we are trying to mimic only occur for a short amount of time each year; when there is enough grass and water for the herds to habitat in an ultra high density. The rest of they year they are scattered to fit feed and water availability, while selectively grazing plants with the highest nutritional value. In nature, availability of water will play at least a big of factor in herd size as feed. By providing an adequate water supply year round, we are allowing them to stay closer together, actually creating more positive animal impact to the soil biology than in nature.
By allowing cattle to graze selectively while migrating through pastures during the dormant season, we are actually creating the conditions to regenerate cool season browse and grasses. At the same time, rather than forcing them to graze non-selectively, your cattle come closer to meeting their nutritional requirements, lowering, or even eliminating the need for supplemental feeding.
Determining when you need the cattle to practice non-selective grazing, or spread out and graze selectively is simple once their instinct to act as a herd is rebooted. They will tell you what is needed as they are grazing. All you have to do is place them on their grazing path and they will instinctively do the rest on their own as you migrate them through your grazing plan.