Forage & Grains
Pasture Quality Variation
Throughout The Grazing Season
By Aimee Hafla, USDA ARS
Added July 30, 2013. It is important for dairy producers and their nutritionists to have an idea of the nutritional quality of the pasture they are providing to their cows. The ideal way to assess forage quality is to gather a representative forage sample from a given area, send it to a commercial lab and wait patiently for a detailed report of nutrient composition. Unfortunately, the cost of forage testing can be a limitation to producers and the time it takes to get results prevents its use to monitor pastures as they are grazed. On-going research projects conducted by the USDA-ARS Pasture Systems and Watershed Management Research Unit in State College, PA can provide examples of monthly pasture quality for grazing dairy farms in this region.
As part of a larger 5-state interdisciplinary research effort funded by the USDA-NIFA- Organic Agriculture Research and Extension Initiative, we are gathering data to evaluate pasture production strategies and the possibility of supplementing winter flaxseed to optimize production while enriching milk composition (omega-3 fatty acids and conjugated linoleic acids). We are starting year 2 of this 4 year project and part of this study involves gathering twice monthly pasture samples from our cooperating organic dairy farms to describe available pasture quality, forage fatty acid composition, and the relationship of milk fatty acid composition to quantity and quality of forage consumed. This methodology has allowed us to begin gathering a robust data set of pasture quality for 14 farms across the northeastern U.S.
The following data, shown in Figure 1 and Table 1, is from one of these farms located in southern PA. This farm milks approximately 55 cows and has 47 acres of permanent pasture and 65 acres used for either pasture or hay. Grazing occurs at night and the cows are generally offered a new pasture each night, depending on forage availability. Forage samples for this study were collected in the late morning from different pastures (some permanent pasture and some hay and pasture fields) at each sampling and are representative of forage quality across the farm throughout the grazing season, instead of within a single pasture. Forage quality for each month is an average of 2 samples from different pastures on the farm. The pastures on this farm contained 62% grasses, 18% legumes, 19% weeds and 1% bare ground, with predominant species including orchard grass, fescue, alfalfa, and white and red clover. Over the 6 month grazing period, crude protein (CP), fiber (NDF) and energy (NEL) of the pasture averaged 19%, 60% and 0.52 Mcal/lb, respectively. As expected, energy levels in the forage were greatest (0.57 Mcal/lb DM; Figure 1) in early summer and early fall and the lowest in October (0.45 Mcal/lb DM). During periods of peak growth in the spring and
fall, CP and NEL are greater, while NDF is lower. During the warm summer months and later fall, growth rates slow and nutritional value of the forage declines. Declining forage quality during the summer is due to high environmental temperatures and low moisture, while later in the fall it is due to lower temperatures and decreasing daylight. The decline in forage quality in July also coincided with the doughty conditions (rainfall deficit of 1.07 inches in July, for this region) that occurred in much of PA in 2012. The greatest drop in forage quality occurred from September to October, and represents a period of time when a negative impact on milk yield would be evident. Calcium concentrations in the forage declined as the grazing season progressed (from 0.82% in June to 0.54% in November), however, phosphorus and magnesium remained steadier (Table 1). The mineral concentrations found on this farm were within the expected range for this region.
Data presented in Figure 2 were collected as part of a one year Northeast SARE funded project evaluating grazing management and pasture productivity. It is taken from a NY grazing dairy which milks 60 cows and grazes 240 acres of pasture and hay land. During the 2012 grazing season cows were supplemented with molasses and balage in addition to pasture due to droughty conditions. In this study the same field was sampled throughout the season during mid morning hours prior to being grazed, and cattle were subsequently moved 2-5 times daily with 45-52 days between rotations. Samples were taken from a low-input hay field that is only grazed after the removal of hay, and where inputs for years have been limited to manure deposited by cattle while grazing. Second cutting was grazed off in July due to the impacts of drought and an extended heat wave. Protein, NDF, and NEL of the pasture averaged 20%, 54% and 0.60 Mcal/lb (DM basis) from July to October. Similar to the previously discussed study, energy in the forage increased 18% from summer into early fall and declined again in October, coinciding with growing patterns and cessation of the very dry period in July.
When balancing diets, sometimes nutritionists turn to book values when an analysis for a particular feed is unavailable. While this approach is generally acceptable for grains, the nutrient composition of pasture (and the subsequent conserved forages that may be harvested) varies greatly depending on geographic location, soil type, environmental conditions, proportions of grasses and legumes, previous pasture use (hay or pasture), foliar feeding, soil amendments and grazing management. The 2001 Dairy NRC over-values protein and energy for both cool season grass and legume pasture when compared to the laboratory results for pasture nutrient composition discussed here. Using values that overestimate pasture energy values when balancing and evaluating the rations of grazing dairy herds will result in less than optimal milk yields even when forage quality is considered high.
Forage quality varies throughout the season, across the farm, and between years. Therefore, in addition to managing the amount of available forage in their pastures, producers must keep in mind that the quality of available forage is not static. Variability in nutrient composition during the grazing season and between grazing years emphasizes the importance of monitoring milk production for indications of decreasing pasture quality and adjusting supplements (if they are being used) and pasture management accordingly. Additionally, this information reinforces that traditional book values may not accurately reflect the nutrient composition of pastures. An update to the Dairy NRC pasture nutrient composition values divided by region is much needed. Nutritionists balancing and evaluating rations for pasture-based dairies should become familiar with pasture quality values for the region, and, more ideally, with the forage quality on the specific farm they are working with.
Finally, it will be interesting to see the forage quality results for 2013. After the drought and high environmental temperatures of 2012, we expect to see comparatively higher forage quality values in 2013, as so far it has been cooler with more normal levels of precipitation. Both of the projects discussed in this article are ongoing, and therefore the results presented are preliminary. We plan to present more detailed findings and a follow-up article in a future issue of the NODPA Newsletter.
Dr. Aimee Hafla is a postdoctoral research animal scientist at the USDA-ARS Pasture Systems and Watershed Management Research Unit in State College, PA. Her current research focuses on extending the grazing season and winter flaxseed feeding to increase profitability and produce high quality component-enriched milk. You can reach Dr. Hafla by email or phone:
firstname.lastname@example.org, (814) 863-0947.