Karen Hoffman Sullivan, USDA Natural Resources Conservation Service
Regardless of the species or class of grazing animal, a management emphasis on maximizing dry matter intake (DMI) from pasture is important. Numerous factors including plant, animal, and human, have an influence over how much pasture forage an animal will consume. The higher an animal’s requirements are, based on production level, the more important maximizing intake becomes. Thus, lactating dairy cows are the kind and class of livestock that are most sensitive to factors influencing intake.
Maximizing pasture DMI is not as critical with beef cattle, sheep, or growing dairy heifers as it is with lactating dairy cattle. Dairy producers see the immediate result of a shortage of DMI in the milk tank. Beef or sheep producers are not looking at gains on a daily basis; they are more likely to be looking at season-long gains on calves or lambs. A few days with limited DMI will be compensated for when pasture is adequate to meet intake needs. Both beef cattle and sheep are influenced by pasture characteristics and grazing behaviors that will be discussed relative to lactating dairy cows.
The key to making a pasture-based dairy ration successful is high dry matter intake, and nothing makes a ration fail faster than a pasture that doesn’t meet intake needs. Most dairymen know that when intake is limited—regardless of feeding system—milk production suffers. This relationship is recognized when feeding in the barn, but often the relationship is forgotten when cows are grazing pasture. If the "pasture feedbunk" is empty, milk production is not optimized. If the "pasture feedbunk" is full, cows will achieve a higher DMI, resulting in higher milk production.
The place to begin looking at feeding grazing animals is in the pasture, because that is where a ration will either succeed or fail. The management of the pastures, as well as the physical attributes of the pasture (too tall, too short, no clover, too much stubble, etc.), are the key to milk or meat production. Regardless of the production goals of the producer, they are more difficult to achieve if the nutritionist hasn’t seen the pastures. If a nutritionist is called by the grazier and asked what to feed, and the nutritionist doesn't or can't go to the farm and see the pastures, the rations for dairy cows on pasture fail. The quality of the pasture is also important, and a pasture sample should be taken as a basis to start balancing the ration.
The components of pasture that we need to be concerned about are plant density, number of tillers/plant, the height of the grass, and species composition. Research from around the world has clearly shown that dry matter intake from pasture is the result of a relatively simple equation. It is:
DMI = intake/bite x rate of biting x time spent grazing
A high plant density results in a higher intake level because the animal can stand in one spot in a pasture and graze from many plants—a high biting rate. A thinner stand will result in a lower biting rate because the animal needs to spend more time walking around and looking for plants to graze.
For example, a first year grazier typically has converted hay fields to pasture. In a hay harvest system the grasses grow much taller, which shades out the clovers and new tiller growth from the existing plants. When converted to grazing, there are areas in the pasture with bare ground or very small plants, limiting both biting rate and intake/bite. Although the paddock may be sized to support a certain number of cows at a certain intake, milk or meat production may suffer because the pasture has not had time to "thicken up." In other words, the pasture feedbunk is not completely full. After 3 years of grazing, however, that pasture will have more plants with more tillers, very few bare spots, and lots of clover, resulting in a very high DMI.
Intake per bite is strongly influenced by the number of tillers per plant—the more tillers each plant has, the more dry matter will be available in each bite the cow takes. Compared to either a total mixed ration (TMR) or stored forages fed individually, a mouthful of grass has less total DM/bite. This is partly due to fresh pasture forage being only 15–25% DM, compared with a typical TMR at 40–50% DM or stored forages such as corn silage at 35–40% DM or dry hay at 85–90% DM. Also, there is more “air space” between tillers at the top of the sward than there is in a TMR, and a higher tiller density results in less air space per bite. Thus, each bite of pasture should contain as many tillers as possible, as a way of increasing the DM contained in each mouthful, which leads to higher overall DMI.
Pasture height also has a strong influence on DMI, and in many cases is the main factor that limits intake/bite. Research has shown that dairy cows generally graze off the top third of the plants, regardless of height. When the sward surface height is around 8 inches, intake per bite is approximately 30% higher than when the sward surface height is only 3 inches. For example, at 8 inches, the cow will graze off approximately 2.6 inches, compared to only .99 inches at the 3-inch height. This is not unexpected when factoring in other sward characteristics, such as number of tillers at each height. Most of the time another animal will graze the same plant (providing it is not tall and stemmy) and take an additional third off the top of the remaining plant mass, leaving an overall post-grazing residual height of 2–3 inches.
There is an upper limit on how much time an animal will spend grazing in a 24-hour period. Grazing time will fluctuate based on reductions in DMI due to bite mass or other factors. There is an upper limit of 10–11 hr/d for lactating animals. Dairy cows need to rest and ruminate, spend time for milking, as well as time for social behaviors. Growing animals will have an upper limit of about 7 hours per day of grazing time, because their total intake is lower than a lactating animal, and they can meet their needs in less time.
There is an influence on grazing time from supplementation, where grazing time is reduced by 3.5–5 min/lb grain/cow. However, total DMI is increased in grazing dairy cows that are supplemented with grain by approximately 7 lbs/d, leading to higher milk production as well.
Before an animal selects a pasture plant to graze, she determines four factors: does it feel palatable, does it smell good, will it taste good, and will it be nutritious?
This is determined through the use of the animal’s nose and facial vibrissae, or “whiskers”, that grow on it. As the nose moves through the pasture sward, the animal “feels” the plants through the vibrissae, sending a signal to the animal’s brain indicating the height where the forage becomes stemmy or where there is residual stubble. This feedback mechanism helps the animal determine which plants to graze based on palatability. If there is a high residual stubble height due to lack of clipping or hay harvest, the animal will not graze lower than the stubble height. The animal prefers to graze material that is soft and easily torn from the plant base. Stubble material is tough, and the animal will generally avoid grazing it in search of something more palatable.
An astute observer of grazing animals will comment they love the sound of grass being grazed by their animals. What many fail to notice is the sniffing and smelling sounds that accompany the sound of ripping grass. As the animal is using vibrissae to determine palatability, she also smells for fouled areas and avoids those plants or areas. Some research indicates she is determining the taste and nutritional value of plants based on volatile gasses released by the plant. This is supported by research showing animals prefer afternoon cut hay over morning cut hay, as they can detect the higher sugar content and nutritional value of the afternoon cut hay.
Grazing animals go through a learning process when encountering a new forage species. If not familiar with a plant, she will only eat a small amount. After a period of evaluation through biofeedback mechanisms to determine nutritional value or toxicity, she will gradually increase her intake of that plant. There is long-term learning that takes place, and once familiar with a plant, she does not go through the learning process again with that plant.
Beneficial nutrients, such as protein, can reach toxic levels and cause animals to stop grazing before full. This has been seen in grazing dairy herds where protein was overfed in the barn. The cows would only graze a short time, and when protein levels from high quality pasture reached toxic levels, pasture DMI would drop off. This has caused losses in milk production, leading to a cycle of feeding more stored forage in the barn. This only further restricts DMI from pasture, and eventually all the benefits of pasturing are lost.
When animals are fed in the barn, it is simple to determine their intake by knowing amounts fed and weighing refusals. In a grazing situation, it is more of a challenge but not impossible. Intake will depend on forage quality. If quality is high, intake will be high. Intake will also be influenced by the quantity available in the pasture and the supplemental feeding program in the barn. Feeding a large portion of the ration in the barn will discourage animals from grazing, especially if they have eaten their fill just before pasture. Many new dairy graziers have made this mistake and ended up frustrated by asking themselves“why won’t my cows graze?”
There are two methods to estimate intake. The first is to measure pasture availability both before and after a paddock has been grazed. The use of a rising plate meter, electronic pasture probe, or a simple pasture stick is recommended. The paddock that is ready for grazing should be measured in several locations to determine the forage available before grazing. The density chart can be used to estimate availability based on forage height and plant density. Forage height determined with a pasture stick should be measured to the top of the sward where the stick begins to be hidden by the herbage. After the paddock has been grazed, measurements should again be made in several locations. Subtraction of the second measurement from the first gives approximation of the total herd intake. Paddock sizes greater or less than one acre in size will need to be adjusted to account for more or less availability.
A second method is to assume the animals can eat to their forage DMI requirements from the pasture. A general rule of thumb for dairy cows is that they will eat 1.8–2.2% of body weight from forage DM. When pastures are managed well, they are highly digestible and 2.2% is more realistic. Research with cows fed only grass/clover pasture has shown that they will eat 3 to 3.25% of body weight. The 2.2% figure is a good starting point when grain will be supplemented. Some cows will eat more than 2.2% of body weight due to their production level or genetic merit. The 2.2% figure means a 1350-pound Holstein will eat 29.7 pounds of pasture DM or 148.5 pounds as fed if it is 20% DM. A 1000-pound Jersey would eat 22 pounds of DM or 110 pounds as fed. After the estimate of forage intake has been determined, any stored forage fed should be subtracted from the total to determine pasture forage requirements.
If the pastures are sparse, the amount expected for them to eat should be reduced based on the density chart above. Also, if there are less palatable grasses or the grass has grown too tall, the estimate of DMI should be adjusted based on observation of grazing patterns in the paddocks. A pre-grazing and post-grazing availability calculation may be most helpful in this case.
Maximizing dry matter intake from pasture can be a complex task. However, it is important to consider all aspects discussed in any type of livestock enterprise, especially if milk or meat production is not being optimized. Developing a checklist with each factor listed may be a first step in determining whether intake is being limited by pasture or animal factors. From there, a management plan can be developed to improve the grazing outcome.