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Kathy Soder, Animal Scientist; Sarah Goslee, Landscape Ecologist
USDA-ARS Pasture Systems & Watershed Management Research Unit, University Park, PA
Matt Sanderson, Agronomist: USDA-ARS Northern Great Plains Research Laboratory, Mandan, ND
Added June 6, 2011.
Pasture management traditionally focused on balancing the quantity and quality of forages for livestock production. This has often resulted in planting a single forage species or simple grass-legume mixtures. Native grasses contain many species of grasslands, nitrogen-fixing legumes and deep-rooted flowering plants (forbs). Today's economic, environmental, and social climate challenges farmers to decrease purchased feed, fertilizer and pesticide inputs, and to increase attention given to soil protection, carbon sequestration, resistance to weeds and insect invasion, and the aesthetic value of the landscape. Can we learn anything from diverse native grasslands that can be applied to managed pasture? Pastures of a diverse botanical composition may possess better persistence, yield stability, and productivity. Managing pasture diversity to include species that fill different roles may offer a tool to aid producers in meeting these new challenges.
Northeastern pastures can be very diverse ecosystems. Research conducted by the USDA-ARS Pasture Systems and Watershed Management Research Unit showed that northeastern pastures contain anywhere from 10 to 70 plant species (average is 30), along with hundreds of insect species. European pasture research revealed hundreds of insects, along with dozens of plants, birds, and other species that play different roles in ecosystem processes like nutrient cycling and food webs.
Much of the research on the effects of forage mixtures on pasture productivity has been in clipped experimental plots, with no grazing. This information has shown that the level of functional diversity (having species that fill different roles, like nitrogen fixers and deep-rooted plants such as chicory) may be more important than the actual number of species in the mixture. Studying clipped plots has given researchers a useful starting point by making it possible to screen many mixtures, but may not offer much insight into the effect of biodiversity on animal production.
A two-year grazing experiment in central Pennsylvania was designed to evaluate the effects of forage mixture diversity on intake, milk production, and grazing behavior of lactating dairy cows. Extensive forage data was collected to evaluate the effect of grazing on pasture yield, pasture quality, plant composition, species diversity, and plant persistence over two grazing seasons.
Four pasture mixtures were established in replicated 2.5-acre pastures at the Penn State Dairy Research Center. Treatments consisted of:
2SP = 2 species- orchardgrass + white clover
3SP = 3 species- orchardgrass + white clover + chicory
6SP = 6 species- orchardgrass + red clover + chicory + tall
fescue (endophyte free) + perennial ryegrass + birdsfoot trefoil
9 SP = 9 species- orchardgrass + red clover + chicory + tall
fescue (endophyte free) + perennial ryegrass + birdsfoot trefoil + white clover + alfalfa + KY bluegrass
Pastures were rotationally grazed with mature, early-lactation Holstein cows from April until August of 2002 and 2003. Herbage allowance was 60 lb. of dry matter (DM) per cow per day. Cows were fed a 13% crude protein (CP) corn-based supplement at the rate of 1 lb. of supplement per 4 lb. of milk with a maximum of 20 lb. of grain supplement (DM) offered, split into two feedings. Cows were moved to fresh paddocks after morning and afternoon milking. Pre-grazing and post grazing herbage mass was measured twice each week during each grazing cycle by hand separating clipped samples before and after grazing.
Nutritive value of all pasture diets was high compared with many similar types of pastures (Table 1). Other research conducted at Penn State with predominantly orchardgrass pastures reported neutral detergent fiber (NDF) of 42-48% and CP of 20-25%. Pastures were not fertilized for the duration of this experiment, otherwise CP values may have been higher yet. However, this also could have changed the species composition of the pastures by favoring the grasses over the legumes.
The NDF of the pastures was lower in the complex pasture mixtures compared to the orchardgrass-white clover mixture (Table 1, page 10). This may have been due to the chicory, which tends to be lower in fiber content than many of the cool season grasses, as well as the increased legume component of the more complex mixes. In general, the total non-structural carbohydrate (TNC) content was higher in the complex mixtures than in the two-species mixture. This makes sense, since as the NDF content decreases, in vitro dry matter digestibility (IVDMD) will increase slightly, providing more energy to the animal. The IVDMD was slightly lower during the second year (2003), perhaps due to cooler weather conditions favoring the cool-season grasses, or perhaps due to maturity of the plants. It is interesting to note the difference in pasture quality between years. 2002 was a dry year (3.4 in. rainfall/month average) in contrast to 2003 (4.6 in./month).
The total diet was approximately 60% forage and 40% concentrate for all pasture treatments (Table 2, Above). Pasture DM intake was not affected by forage mixture. These pasture DM intake values may seem a bit high compared to commonly used parameters for pasture intake of grazing dairy cows. Chromic oxide was used as a fecal marker to estimate pasture DM intake in this study, and it is widely recognized that this technique may overestimate DM intake by approximately 10%. However, at this time, it is one of the best methods we have to estimate intake of grazing cows, where measuring weights of feed offered and refused is not possible.
Milk yield, fat and protein were not affected by forage mixture complexity (Table 3). These were high-producing dairy cows and maintained high levels of milk yield throughout the trial. Some individual cows were producing well over 100 lb. of milk daily during the early part of the trial. It might seem surprising that milk fat levels were not depressed, despite the low NDF levels in the pasture. We anticipated these pasture nutritional values, and tried to compensate for them with the by-product feeds included in the grain ration (Table 2, footnote) to provide the needed fiber requirements. Milk urea nitrogen levels were slightly affected by forage mixture, but all values were within normally reported ranges for pastured cows and did correspond to the CP values of the pastures. The conjugated linoleic acid (CLA) content doubled while grazing when compared with the pre- and post-trial samples (TMR diet; Figure 1). Cows grazing the 3, 6 or 9 species pasture had greater CLA than the 2-species, possibly due to the increased unsaturated fatty acid content of the chicory.
Figure 1. Conjugated linoleic acid (CLA) content (g CLA/g fatty acids) from cows grazing 2, 3, 6, or 9 forage species (averaged across both years).
Pasture Growth/Composition Results
In 2002, forage yield was significantly lower on the 2SP mixture than the more complex mixtures (Figure 2, above). In 2003, with greater rainfall, there were no significant yield differences among mixtures. The largest increase in forage yield in 2002 occurred between the mixture that did not contain chicory (2SP) and those that did (3SP, 6SP, 9SP). Weed proportions were similar for the 2SP and 3SP mixtures, whereas the 6SP and 9SP mixtures had lower weed populations than the simple mixtures (Figure 3, below).
Figure 2 (above). Seasonal pasture yields (lb/acre) of pasture seeded with 2, 3, 6, or 9 forage species.
Figure 3 (above). Weed percentage of pasture seeded with 2, 3, 6, or 9 forage species (averaged across both years).
Although increasing the complexity of the forage species mixture did not improve productivity or intake of lactating dairy cows, the cows all maintained high levels of production. The complex mixtures (3SP, 6SP, and 9SP) were more productive than the simple orchardgrass-white clover (2SP) mixture during drought, and also had reduced weed pressure. These results suggest that managing for a moderately complex (3 to 5 forage species with different functional roles) mixture of forages on pasture may result in greater carrying capacity of the pastures due to increased forage productivity and reduced weed competition, while maintaining animal productivity.
Dr. Sarah Goslee is an Ecologist at the USDA-ARS Pasture Systems and Watershed Management Research Unit in University Park, PA. She has been in her current position since 2002 where she studies the factors controlling plant species diversity in managed grasslands.
Dr. Matt A. Sanderson is the Research Leader at the USDA-ARS Northern Great Plains Research Laboratory in Mandan, ND. From 1996 to 2010, he was a research agronomist and lead scientist with the USDA-ARS Pasture Systems and Watershed Management Research Unit in University Park, PA where he did research on forage and pasture management, warm-season grasses for conservation and bioenergy, and grassland diversity.
Dr. Kathy Soder is an Animal Scientist at the USDA-ARS Pasture Systems and Watershed Management Research Unit in University Park, PA. She has been in her current position since 1998 where she conducts research on the nutrition and grazing behavior of pasture-based dairy and livestock.