Here’s the third part of my notes from Woody Lane’s lecture on grass. This is intuitive once you say it and graph it; but grass, when it first starts growing, has the highest nutritional content it’s going to get. It’s TDN (total digestible nutrients) at this stage is likely in the low 70’s, percentage-wise. As it grows, it increases in volume, of course, but it also starts to decrease in TDN. When it’s headed out to seed, its TDN is down around 45%. So, there is an inverse relationship there, between increasing volume and decreasing nutritional value.
The TDN curve, over time, looks like this:
First cutting hay, or spring grass, has the steepest TDN curve, it can lose up to 0.5% TDN per day as it’s racing to go to seed. So, this is why the timing of cutting and baling first-cutting hay is dicey- every day delay is sliding you further down that curve . If you wait until the bottom of this curve to bale, your hay is no better than "old straw,” according to Woody’s numbers.
After a grass field has been cut or grazed once, the second growth curve is much gentler and more forgiving. You have a lot more time leeway in which to graze or cut feed while it still has good nutrient quality. That is why people tend to value second cutting hay. Though it’s not better, per se, is it’s just less likely to have been cut too late, so more likely to have good nutrition content.
Stressed-out grass will try to go to seed earlier than normal, so the curve gets shifted to the left and stays steep. Grass can be stressed by lack of nutrients in the soil, temperature extremes, or too much or too little water.
Gentle grazing, on the other hand, pushes the curve to the right- it delays the grass being able to go to seed.
How Grass Grows
If you think of grass blades as little solar panels capturing energy, it’s easy to realize that young plants don’t yet have many solar panels, so they grow slowly at first. This is Phase 1 growth. As the grass starts to take off, it follows a steep near-linear curve where it’s adding mass rapidly; this is Phase 2. Phase 3 is when the grass growth tails off, it puts all of its energy into its seeds, and then it goes dormant. We ideally want to put animals on the grass late in Phase 2, when volume is high, but while nutrition is also still good. And we want to take them off at the bottom of Phase 2.
If we remove animals too late, we’ve put the grass back into that slow-growth Phase 1 state, and have added a delta of recovery time for the grass to get back into Phase 2. If we put animals in a pasture that’s headed into Phase 3, they’re going to pick through it to find what little tasty grass is left, and devour only that. This will tend to leave behind less desirable species of grass and weeds to thrive and re-seed, and will kill the most desirable species that are staying green the longest. Over time, this will ruin a pasture’s desirable species diversity. You’ve seen pastures like this, I’m sure: what little grass is left is a millimeter high, and the rest of the pasture is filled with thriving, tall thistles or some other noxious weed.
There is one theory of grazing that we should mimic what the buffalo once did, which was to graze grass down to nothing before moving the animals. Woody confirmed that this is a valid method, but that method prescribes not retuning to that section again for a full year. This is what the buffalo would have done, since they ranged so widely. So, it’s only practical when you have lots of land in comparison to the number of animals grazing; and is not feasible for most people. And, I think this also only applies to some species of grass in some climates- i.e. the environment of the plains. If we only grazed our grass hard only once per year, in the Northwest, it would be a jungle most of the rest of the year!
But, the most important consideration of all is re-growth. Woody asked the audience, “how long does it take grass to start re-growing?” People hemmed and hawed, thinking they did not know the scientific answer. He persisted, “c’mon, you mow the lawn- how long does it take before it starts to grow again?” Ah, now we get it, yeah, just a few days and it’s already re-growing, right? So this is the golden nugget: if you leave animals on a section of grass longer than about five days, they are going to notice that there is tender new grass growing on the plants they’ve already grazed down. And they’re going to eat that! If there is still older, taller grass left in the square, they’ll bypass it, because the re-growth is yummier, right? So now they’re abusing some of the grass plants, which are going to have a hard time getting back into Phase 2, and may even die. So we have to prevent this from happening.
Staying in the Middle of the Curve, By Height?
The goal is for animals to be eating the grass (or cut hay) along the middle of the TDN curve and inside of Phase 2, where you are balancing the optimal values of volume, nutrition, palatability and re-growth potential. We’d like to time it perfectly so that in five days or less, they eat the section down to the bottom of Phase 2-uniformly, and then we move them out.
So, how to stay in the middle of that curve? Woody reminds that the typical magazine article advises to start grazing at x inches, and stop grazing at y inches. This describes what’s happening in the growth curve, sure. But it doesn’t take into account re-growth timing, and it doesn’t jive with the math we use to quantify animal intake, which is usually defined in dry weight. Knowing approximately how much dry weight each sheep eats, how can we be sure the herd can eat the square in five days or less, based on the number of animals we have?
There is no standard conversion factor from inches of grass to dry matter pounds, because it depends on your grass! In New Zealand, the recommendation is to assume 1,000 lbs of feed in the first inch, and 500 lbs of feed for every inch thereafter; so 5” grass would contain 3,000 lbs per acre. But New Zealand has very fertile soil; by comparison, grass in our region could offer as little as 500 lbs for the first inch, and 200 lbs thereafter; rendering only 1,300 lbs per acre. So, inches of grass will not do when figuring out how to manage your grazing rotation. You will not be able to ensure your animals can eat the square in less than five days.
Next I’ll talk about Woody’s method for accurately estimating your grass mass, or weight, and planning your 5-day grazing squares around that.