“A lot of livestock producers like playing with the animals but forget to manage the forage along with them,” began Jeff Lehmkuhler at this year’s Beginning Grazing School held in Versailles, Ky.
The University of Kentucky extension beef specialist noted that forage factors often regulate an animal’s dry matter intake, and one of the first things to consider is plant maturity. “Forage is always going to be higher quality when it’s in an immature, vegetative state. That’s when we’ll have a higher ratio of leaves to stems. Once a forage crop reaches the reproductive stage, there is no longer leaf development as the plant will prioritize seedhead development.”
The long-tenured beef specialist explained that forage has to be reduced to a small enough particle size to exit the animal’s rumen. If it isn’t reduced, then it stays in the rumen. This is what occurs when animals are forced to eat extremely mature forage. As a result, the rumen stays full, and dry matter intake drops. “This phenomenon is a big reason why energy intake is the most common limiting factor in many forage-based livestock systems,” Lehmkuhler asserted.
Two other factors that impact dry matter intake are the age or size of the animal and its stage of production. An animal that isn’t lactating doesn’t have the same metabolic demand compared to the one that is, so it doesn’t eat as much. Stage of animal production and plant maturity also interact with each other to impact dry matter intake (see table below).
“It’s important to think about these relationships when determining carrying capacity,” Lehmkuhler said. “You don’t know how many calls I get from people who just bought 10 acres and plan to make a living grazing 30 cows. It’s not going to work.”
Seasonal differences
Forage growth, unfortunately, isn’t constant from March 1 to November 1. “Taking this into account will prevent you from having a pasture that looks like a putting green on June 1 and having bare ground by July 1,” Lehmkuhler cautioned.
To hit dry matter intake goals, pastures need to be kept vegetative as much as possible. Lehmkuhler suggested increasing the speed of rotations in the spring. For example, one full spring rotation may take 14 to 21 days, while a full rotation of animals grazing each paddock during mid-summer might be extended to 30 to 40 days.
During spring, Lehmkuhler recommended to start grazing when pastures have 3 to 4 inches of growth. Even then, it can be beneficial to also feed some hay, which helps ensure adequate dry matter intake. “If cows are turned out too early, they can end up walking the fat off their backs as they try to get enough to eat.”
On the flip side, waiting too long to turn cows out on spring pastures can make it difficult to keep the last paddocks from getting too mature. This why some farmers make hay from those paddocks the first time through in the spring and then put them back in during subsequent rotations.
In Lehmkuhler’s experience, forage quantity is often more limiting than forage quality. Maximum dry matter intake for cattle occurs when pastures reach about 2,000 pounds of dry forage per acre, or roughly 10 to 12 inches of forage height. Conversely, dry matter intake becomes limiting when forage height is less than 3 to 4 inches, or below about 1,200 pounds per acre. “On the lower end, for every bite that they take, they aren’t getting as much to eat,” Lehmkuhler noted.
From research studies, cattle have been shown to graze about eight to 10 hours per day. “Grazing time and biting rate are relatively constant,” the beef specialist said. “Bite size varies with amount of forage available, and that impacts dry matter intake.”
Lehmkuhler emphasized to leave enough residual growth so there is adequate green tissue for the plant to photosynthesize and rapidly regrow. He noted that overgrazing pastures is still the most common mistake that graziers make. This situation is both detrimental to the pasture and to animal performance.
