Just like harvested forages, nutritional quality of pastures is related to the maturity of the forage when harvested. Dairy producers strive to achieve legume forage with 20 to 23% crude protein (CP), 26 to 30% acid detergent fiber (ADF), 38 to 42% neutral detergent fiber (NDF), and a net energy for lactation (NEL) of 0.62 to 0.68 Mcal/lb. Typically, the ADF content of the pasture is more closely associated with energy and is often used to calculate NEL content, while NDF is associated with intake and rumen health. Smaller databases for pasture compared with stored forages can mean less accurate values, particularly energy values.
With good grazing management, grasses harvested by cows will be in a vegetative state and approximately 6 to 8 inches tall, depending on the type of grass. Grasses harvested as hay or silage are typically more mature than when harvested as pasture, thus pasture should be of higher quality than stored forages. Likewise, legumes such as clover or alfalfa are usually grazed at an earlier stage of growth than when harvested as stored forage. Therefore, with excellent grazing management we should expect to have higher quality forage when harvested as pasture than when harvested as stored forage.
Pasture quality will depend on many factors, including:
- Geographic location
- Environmental conditions (temperature, humidity, precipitation)
- Type of grass and/or legume
- Grazing management
Seasonal Forage Growth
This table shows average nutrient composition for several types and mixtures of high quality pastures. These are average values and should be used as a guide - forage testing and monitoring of nutrient composition are needed to more closely monitor the supplemental feeding program. In general, pastures containing some legumes are higher in nutrient value and will likely be consumed in higher amounts than straight grass pastures. The type of grass and the inclusion of legumes will influence pasture composition. The seasonal effects on nutrient composition are illustrated with columns titled spring, summer, and fall. For most grasses and grass/legume mixtures, the protein and energy content are higher in spring and fall and lower during the summer.
These fluctuations in nutrient content are closely correlated with the annual growth cycle of the forage. Figure 1 graphically depicts the production cycles of cool-season grasses, forage legumes, and warm-season grasses. For cool-season grasses (our most common pasture types), the majority of growth occurs in the spring time and into early summer. When temperatures increase and precipitation decreases in summer, those same cool-season grasses typically decrease in production. A smaller fall flush usually occurs that is a result of more moderate temperatures and greater precipitation.
Figure 1. Growth curve of various types of pasture plants throughout the growing season. (Stephen K. Barnhart, extension agronomist, Iowa State University Extension service).
Forage legumes follow a similar pattern with one major difference. The legumes tend to maintain a slightly higher productivity level during mid-summer. One reason for this is that legumes establish deeper root systems than grasses, and can utilize greater amounts of soil moisture. Peak production for warm season grasses occurs during mid-summer, when the other forages decrease production; however, warm-season grasses are not typically recommended for lactating dairy cows due to their high fiber and low energy content in relation to animal nutrient requirements.
Table 1 shows the change in nutrient composition of a mixed mostly grass pasture in the Northeast US. If we compare the numbers in Table 1 with the graph of cool-season grasses in Figure 1, some interesting trends can be noted. During periods of peak growth (spring and fall), nutritional value (CP, rumen degradable protein (RDP), non-fiber carbohydrates (NFC), and NEL) is higher, while fiber content (NDF) is lower. The reverse happens during the summer months, when growth rates are lower - fiber increases and nutritional value declines.
Table 1. Variation in nutrient composition of a mixed mostly grass pasture throughout the grazing season.
|Adapted from Rayburn, 1991.|
|RDP, % of CP||74||73||72||71||71||74|
Table 2. Nutrient degradation of b fraction (insoluble, potentially degradable fraction) of various pastures species.
|b1 = Insoluble, potentially degradable fraction. |
c2 = Rate of degradation (%/hr) of the fraction.
The values presented in Tables 1 and 2 can be important when formulating dairy rations around pasture. The variability in nutritional composition throughout the grazing season or between grazing years emphasizes the need for re-formulation of the dairy ration throughout the grazing season. In addition, regular forage testing is important as the nutritional quality changes. When formulating rations with stored forages, forage tests are usually taken when a change in bunkers occurs, when the field harvested changes within a silo, or when a new source of forage is fed. The same applies for pastures.
During periods of summer heat and drought, additional forages may need to be provided to lactating dairy cows. However, at other times of year, perhaps only concentrate, mineral, and/or a rumen undegradable protein source is all that is necessary to maintain animal health and productivity. One of the most important concepts with grazing is to frequently monitor forage quality, especially the first few years, and to be flexible and be prepared for change, depending on weather patterns, forage growth, and changing nutritional composition.
The nutrient composition of an orchardgrass pasture during a grazing season from a Penn State study is shown in Figure 2. These results are from well-managed pastures that were grazed at a stocking rate of 1.3 cows per acre and had nine grazing rotations. Quality remained high with this well-managed pasture where grass was grazed in the vegetative state and should be representative of intensively-managed pastures. Crude protein remained greater than 20% for the entire grazing season. Fiber increased and in vitro dry matter digestibility (IVDMD) decreased during the summer. These results suggest the analysis of pasture perhaps three times per year to coincide with changing seasons may be adequate when pasture availability is not limiting.
Figure 2. Nutrient composition of orchardgrass-based pasture during six month grazing with 9 rotations per paddock.
If cows are moving to pastures that have a different forage composition, forage testing is recommended. Keep in mind that forage tests are always subject to errors and good judgment is necessary when interpreting results and developing feeding strategies.
Forage Quality as Related to Maturity
- Varies depending on time of year and stage of growth cycle (Figure 3)
Figure 3. Changes in forage quality components during various growth stages of the forage.
As pasture matures:
- Leaf % decreases
- Stem % increases
- Protein and energy decrease
- Fiber and lignin increase
- Pasture intake decreases
Pasture Protein - A Balancing Act
Although the total protein in well-managed pastures is high, the protein is high in ruminally degradable protein (RDP). Often, 70 to 80% of the protein in pasture will be degraded in the rumen, whereas the recommended requirement for RDP in the total dairy ration is between 62 to 68% of the total protein. Providing ruminally available carbohydrates, primarily from concentrates, but also from other forages, will help animals utilize the high levels of RDP in pastures more effectively. If non-structural carbohydrates or energy are lacking in the diet and rumen, the high ruminal RDP in pasture will result in high levels of rumen ammonia, which is converted to urea. This urea then appears in blood and milk, with much of it eventually in the urine. Thus, the high RDP in pastures is often wasted by the cow. High levels of urea in the blood have been linked to lower reproductive efficiencies and excretion of urea requires energy for the animal to excrete.
Keys to High Quality Forage Production
- Know when canopy is ready to be grazed
- Know how much stubble residue to leave before moving to another paddock
- Know how long it takes to use the canopy to the desired stubble height
- Use a stocking density which allows quick removal of forage and uniform distribution of excreta
(Source James T. Green, Jr. 1996 PA Grazing Conference Proceedings)
Well-managed pastures, regardless of the plant species, can be high in nutrient quality and often exceed the nutrient composition of high quality stored forages. Good grazing management is essential to maintaining quality pasture. Changes in season trigger changes in rate of plant growth and subsequent changes in nutrient composition. The continual changing of pasture quantity and quality during the grazing season provides challenges to producers using a grazing system. Use of available information about forage quality and nutrition can lead to sound grazing and feeding management decisions.
Sampling Pasture for Nutritional Analysis
Obtaining a representative pasture sample for nutritional analysis can be challenging. Animal selectivity plays a large role in what specific plants and plant parts the animal consumes, and plant composition, soil fertility, soil type, and moisture can vary across even a small paddock. The following is a recommendation for sampling pastures:
- Sample a pasture when animals are first allowed access to it (before much grazing occurs).
- Observe the animals-take note of which plants they consume/avoid, and to what height they are grazing. Accurately determining how much of which forage to sample can be difficult. With a little practice, an experienced manager can accurately identify the species being consumed at the time of sampling.
- Walk around the entire pasture with a container such as a clean 5-gallon bucket and sample randomly from a minimum of 25 to 30 locations throughout the entire pasture (sample numbers may vary depending on pasture size).
- To take a grab sample, grasp a small handful of grass, similar to a cow when it wraps its tongue around the forage. Avoid dung piles, areas near watering troughs, weedy patches, and other areas that animals tend to avoid when grazing.
- Tear the plant at the grazing level (usually the top 1/3 of the plant). Discard any roots, soil clumps, or lower stems that may have pulled out with the sample.
- Thoroughly mix the collected forage and take a representative sample for analysis-fill a gallon sized plastic bag with the representative sample.
- It is important that pasture samples be immediately frozen to prevent marked chemical changes such as fermentation. The most practical alternative is to pack the sample tightly in a plastic bag, exclude all possible air, freeze, and promptly mail to the laboratory. With good container insulation, the sample will arrive in a cool condition or with a minimum of a silage-like fermentation.
- If different paddocks differ greatly (slope, plant composition, soil type, sun exposure, etc.) sample them separately, as nutritional qualities may be quite different.
Shortcomings of Pasture Sampling
- Pasture samples only provide an idea as to the likely quality of an animal's diet. These samples are not an exact representation of the animal's diet.
- Because we usually sample a single species, it must be remembered that other plant species comprise cattle diets. Analysis on the sample may indicate a deficiency, sufficiency, or excess in requirements, however other diet components may compensate and either improve or reduce diet quality.
- Cattle selectively graze, and therefore the plant material we sample maybe untouched by cattle in favor of other plants.
Pasture sampling is a useful aid to gauge the likelihood of pasture quality and in turn determine whether nutritional requirements are being met. It must be remembered pasture sampling is not an exact science.
Published as pages 7-15 in proceedings from "Nutrition of Dairy Cows on Pasture-Based Systems" held March 31, 2003 in Grantville, PA.