Plant biology illustrates how grazing impacts pasture health and managementWritten by Natasha Wheeler
“Plant biology basics have a direct application to pasture management decisions that we make,” notes Laura Kenny, program associate in the department of animal science at Rutgers University.
Looking at grass, the youngest leaves can be found near the ground, shooting up out of the middle of a plant, with older leaves sheltering them around the outside and bottom.
“The older leaves have already been produced. They are just getting wider and longer,” she says.
In a broadleaf plant, the main growing point is found at the top of the plant, and the younger leaves are found coming out of the center, away from the ground.
“Leaves at the bottom are larger and older. If we mow this pasture and cut both of these plants in half different things happen,” she continues.
When the tops of grass plants are cut off, the most important growth point remains intact, whereas the broadleaf plants lose that growing point when the top half is cut off.
“What is the grass plant going to do? It’s going to shoot out more leaves, and it’s going to elongate the leaves that we’ve shortened,” she explains.
The broadleaf plant, on the other hand, can no longer produce leaves on that stem. It must use energy reserves from the roots to grow a whole new stock.
“That takes a lot of energy out of the plant,” Kenny says. “This is one of the reasons we recommend regular mowing in pastures. When we mow a grass, we generally get more leafy grass and more nutrition for our livestock. It will also knock down the growing point of broadleaf weeds. Eventually, we’re going to diminish their energy reserves in the roots and, hopefully, eventually kill those weeds.”
Young versus old plants
Considering vegetative and reproductive growth in plants can also affect the impacts of grazing.
“Vegetative growth is soft growth. It’s leafy, and it would make good quality hay if we were buying hay. Reproductive growth is where we start to see seed heads, and the plant gets more fibrous,” Kenny describes.
Nutritionally, younger plants, in a stage of vegetative growth, have a higher sugar concentration. Older plants, which have matured into reproductive growth, typically have higher fiber and lower sugar content.
“Another thing to note is that the growing point is going to rise in the reproductive plant as it starts to produce seed heads. If we cut that plant in half, we are going to remove the growing point, and the roots are going to have to use their energy to send up a new stock,” she adds, discussing the mature grasses.
Photosynthesis, another component of plant biology, is the process responsible for turning carbon dioxide from the atmosphere into carbohydrates used for plant growth.
“This process happens primarily on the leaf’s surface – not on the stem, not on the seed heads, just on the leaves. Now, what do our horses like to eat when we turn them out on nice pasture? They go for the leaves,” Kenny remarks.
When livestock graze, they reduce the photosynthetic capability of plants, and more leaves removed by grazing means the plant has a reduced ability to regrow after livestock are taken out of the pasture.
“This takes us to the take half, leave half rule. We see we don’t have any problems until 50 percent of leaf volume has been removed. Once we remove over 50 percent of the leaf area, the photosynthetic area, it’s going to really injure the plant, and it will take the plant a lot longer to recover,” she comments.
The difference between 50 and 60 percent of removed leaf matter is also significant. In one example study shared by Kenny, root growth is stunted by two to four percent when 50 percent of leaf matter is gone versus a 50 percent loss of root growth when 60 percent of leaf matter is removed.
“That’s a pretty bad injury for the plant, and it’s going to take a long time to recover. That is why our take half, leave half rule is very important in grazing,” she states.
To better understand grazing, it is also helpful to know what species are dominant in a given pasture.
“Taller grasses like orchard grass, brome and timothy will grow up to six to 10 inches and possibly even taller. If our grass is about six inches tall, we want to stop grazing it at about four inches high. However, if our pasture is made up of Kentucky bluegrass that maxes out at four inches, it can be grazed a little bit lower before we stop grazing. We will still have a healthy plant that can recover once it’s given some time,” Kenny explains.
The rate of growth may also have an impact on how grasses react to grazing, as there is very little plant growth between about zero and three inches tall.
“This is because there is very little leaf area for photosynthesis. All of the energy for this growth is coming from the roots. However, once we reach about three inches and start growing up to about 10 inches, the plant is growing its leaves and making its own energy. There is exponential growth here,” she says.
Once plants reach 10 to 12 inches tall, they progress into reproductive growth, and energy is reallocated from growth potential to seed production and reproduction.
Lastly, Kenny notes that the seasonal pattern of pasture production also impacts grazed grasses. Growth rates are often higher in the spring with adequate moisture, slower during the high temperatures in the summer and possibly higher again in the fall if there are moderate temperatures and rain events.
“To recap, grass grows from the ground up, photosynthesis is critical for regrowth of a plant that has been grazed, the fastest growth is between three and 10 inches tall, and production will likely be highest in the spring,” Kenny concludes.