Flipping through pages of popular press magazines or webpages, we have all sorts of discussions about which methods are the best, some novel and revolutionary grazing management technique, many claiming impressive and sometimes unrealistic results. No matter what you do, how frequently you move your cattle or how long you leave them in a pasture, the principles to follow are simple and the same everywhere.
It is important to understand those principles and adapt to your situation. Matt Poore recently published a great article in the April 2020 issue of Progressive Forage (Managing your stocking rate) about the importance of carrying capacity and stocking rate. (Read that first if you haven’t yet.) In short, if the balance of how much forage you can produce and how much forage you need is negative, you are in trouble – and there is not much else you can do. Fix that first. To build up on those concepts, we present a plant and animal perspective on forage utilization and how it affects plant growth and animal performance.
From the plant physiology perspective
First, have in mind that plants have two sources of energy: photosynthesis, by capturing sunlight through the leaves; or stored reserves (e.g., crown, stems, roots, rhizomes, bulbs). Figure 1 shows the classical “sigmoid” curve of plant growth, representing time (or the equivalent to biomass in a pasture) and the rate of growth.
Recently planted or overgrazed pastures are in the lag phase, where the low leaf area limits light interception and thus photosynthetic capacity, resulting in slow growth.
Furthermore, the shallow root system limits access to nutrients, and the bare ground represents not only waste of sunlight but an opportunity for weeds. In perennial pastures, plants will use stored reserves to compensate for the lack of photosynthetic capacity in that initial phase of regrowth. This means recurrent overgrazing will result in depletion of reserves and ultimately decline in pasture stand. As plants grow and cover the ground, light interception increases and – given availability of nutrients – they get into the maximum (exponential) growth phase. At this point, we have the best compromise between quantity and quality, along with the maximum pasture production.
As plants keep growing and start self-shading, old leaves will senesce, resulting in a drop in net accumulation and reduction in quality. At this point, some plants will start the reproductive phase, further dropping quality and slowing down or stopping leaf growth. There is a great accumulation of stems, dead leaves and increase in fiber, reducing digestibility and protein concentration. When allowed to grow into reproductive stage, many annual forages (e.g., pearl millet, oat, rye) will not recover completely from grazing or mowing, meaning that the pasture is pretty much finished producing.
From the grazing animal perspective
Imagine two scenarios: a recently planted growing pasture or a pasture which you are returning to graze. On a growing pasture, in the beginning we have tender, high-nutritive-value forage. However, the amount of biomass is very limiting. This reduces the size of the bite, and the animal cannot harvest the necessary amount of forage in a day. Very good quality but not enough quantity.
As the pasture grows, we reach the middle point, where we have the best compromise between quantity and quality. The canopy is dense, allowing for big bites of fresh forage, and the animals can eat sufficient forage in less time. In other words, the animal harvests only the best (leaves).
As we move on, the pasture grows more mature; we see more stems, then dead leaves and seedheads appear. At this point, the quality drops, and the animal starts sorting more, which means more time grazing to achieve the required daily intake and more forage “wasted” by sorting and trampling.
In a second scenario, consider a pasture that has been well managed and we are back to graze it at the top of that exponential phase. Now let’s divide this pasture canopy in three layers: top, mid and bottom. The top part of the canopy has the most leaves, and the animals will move along the pasture, harvesting mostly that top layer. As animals move into the second layer, which is composed of a mixture of stems and leaves, quality declines some but there is still a good amount of biomass, allowing for big bites.
Then animals move to the bottom layer, which is composed basically of stems and dead leaves, with lower density of forage (sparser). At this point, the animal not only has low-quality material to eat but struggles to harvest that forage, and this has a large impact on performance.
As a rule of thumb, as you look at the pasture and you see all that top layer has been grazed, the animals are already about halfway through the second layer. It is time to move on.
Figure 2 presents two grazing management approaches, a more frequent and lenient (fast rotation and 40% of initial biomass removed) versus a “traditional” rotational system, where we target harvesting efficiency (80% removal).
After grazing, the large amount of residual leaf area on the lenient management (left) permits a fast regrowth (higher daily herbage accumulation rate) which, at the end of the season, also results in more total biomass produced. Furthermore, since animals are only “eating the best,” performance is higher and compensates for the lower stocking rate.
From an ecological perspective
Heavy grazing is detrimental in all fronts and, other than for very specific situations (to stage a pasture for overseeding, for example), it is not recommended in any case. As mentioned before, it causes depletion of plant reserves, resulting in deterioration of the pasture. The open spaces can result in nutrient losses, erosion and offer an opportunity for weed encroachment. In “mob grazing” systems, where little residual plant biomass is left after grazing, the excessive amount of readily available nutrients from excreta can be lost because plant growth is slow.
Ungrazed or undergrazed pastures can also “slow down” nutrient cycling processes, resultsing in high accumulation of combustible material, increasing chance of wildfires, and in diverse grasslands can result in loss of biodiversity as one or few plants tend to dominate.
So, what to target?
Forage left behind is not wasted. We need to leave leaves behind so the plants can recover and so we don’t offer our animals low-quality forage. The bottom line is: Long regrowth intervals and intense grazing will negatively affect your animals and your pasture. Our target should be to be in that middle range in Figure 1, where we have the best compromise between quantity and quality, which means high frequency and moderate grazing intensity. As a rule of thumb, target graze the top half of the pasture and leave the rest behind, ensuring fast plant regrowth and best animal intake and performance (Figure 2). Just as a general guideline, Table 1 shows some targets for grazing management for common forage species.
Finally, use this strategy for any stocking method. Even more: If necessary, use a combination of methods in your farm according to your resources’ availability, animal category and forage species.
Jean Savian is a researcher at the National Pasture and Forage Research Program, National Institute of Agricultural Research in Uruguay.
- Assistant Professor, Forage Extension Specialist
- University of Florida – Gainesville
- Email Marcelo Wallau