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Pasture testing, NIRS as tools for dairy grazing management

Patty Laskowski Morren and Thomas Griggs Published on 06 February 2015

Dairy cows grazing

Grazing offers an alternative animal management and forage harvesting system that contributes to economic, environmental, lifestyle and other producer goals.

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Pastures offer lower cost per pound of dry matter than harvested forages and opportunities to reduce machinery use, produce high-quality feed from some sites that are not well-suited to mechanization, build soil organic matter and meet organic production requirements.

For a dairy, pasture can be an integral part of the cow ration – and especially for certified organic dairies, which are required to furnish 30 percent of the ration from pasture during the grazing season. Pasture nutritive value can easily exceed that of harvested forages.

Why test pasture?

Testing pastures accomplishes two objectives: to monitor soil nutrient availability and plant uptake and to test standing forage to manage animal nutrition.

Plant sampling goes hand-in-hand with soil testing for efficient and effective nutrient use and application. Testing can also inform producers of nutrient deficiencies or excesses that may be causing other problems.

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Plant tissue samples to complement soil sampling should be taken at the same time and growth stage every year. Guidelines for soil and tissue sampling from the University of Wisconsin, Madison Soil and Forage Laboratory are available online.

Sampling pastures for animal nutrition will be the primary discussion here. First, a baseline of data should be established and then further comparisons or changes can be made over time. With a baseline, the producer can identify how well the pasture is likely to be meeting nutritional requirements of the herd at points in the season.

How to sample

Sampling pastures is similar to that of harvested forages in the need to obtain a representative sample. A critical difference is that a pasture sample should approximate what animals actually select in a grazing situation. In addition, handling and sending fresh material to the lab is different than for harvested material.

First, identify an area to be sampled, typically a field or grazing unit. Walk the area in a zigzag pattern and stop every 20 to 30 feet or consistent number of steps so that a total of 20 to 50 collections are made from that field.

At each stop, reach down and “grab” a small sprig of forage between the thumb and forefinger and pull it off at the height livestock will graze. Select what livestock will eat, avoiding dung deposits and weeds that will be refused. Place each collection into a paper or plastic grocery bag and then mix the composite.

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Since fresh forage is 75 to 80 percent water and low in density, a fresh sample equal to the volume of a gallon resealable bag at least half full will yield an appropriate dry sample for analysis. Begin drying the sample or freeze it immediately.

To dry, spread on newspaper or a screen and place in the sun, a food dehydrator at no more than 140°F or near a fan. After the sample is dry, place it in a plastic bag, remove air, seal it tightly and send it to the laboratory. If the sample is not dried, it should be delivered to the laboratory frozen or on ice.

Developing a baseline

Sample each management unit at the beginning, middle and end of the grazing season. Alternatively, sample each grazing unit right before or at the beginning of each grazing cycle, which may result in five to six samples per unit per year.

Sampling throughout the season and recording sample descriptions and results for each management unit will establish growth rate and nutritional profiles of the grazing units. Development of a photo album with corresponding forage test results can serve as a future decision-support guide.

Sampling should continue for three years in order to establish normal ranges for a farm. After this time, sampling can be done less frequently as necessary to monitor the system and during unusual conditions such as drought.

What other circumstances suggest a need for pasture sampling?

Sampling can identify variation within fields during the growing season and across years, between varieties of the same species and between fields. Testing for constituents, such as digestible fiber or sugar, may provide further information and will help balance the ration better.

Variations in constituents affect how feed is utilized by the animal. For example, two samples from different varieties may have similar neutral detergent fiber (NDF) and protein, but if one has higher NDF digestibility, there will be more energy available for the animals and better production.

Pasture testing can also evaluate new products including new varieties and mixes of forages or annual species and crops such as brassicas grown to extend the growing season.

Pasture sampling results can support feed and mineral supplementation decisions. Harvesting by grazing targets high-quality diets of young leafy plants, which will be relatively low in highly digestible fiber as well as relatively high in protein.

Here, supplementing with corn silage is a good choice because it balances the high protein of the pasture with the lower protein of corn silage while at the same time adding energy through grain in the silage.

But what if these high-quality conditions are not met because weather or other conditions do not permit harvesting fields or paddocks to keep ahead of the grazing animals? Growth may accumulate beyond ideal high-energy stages, including shifts to reproductive condition, and this in turn could limit animal performance. This can happen in late spring when high pasture growth rates result in surplus forage.

Changing animal numbers to balance supply and demand, using appropriate livestock classes, or mechanically harvesting surplus forage may be options. In these situations, testing pasture is important in order to capitalize on available pasture resources.

Pasture becomes another component in developing a balanced ration, and other ingredients can be adjusted in order to add protein, energy, mineral and possibly fiber sources.

Similarly, fall-stockpiled forage for extension of the grazing season can lose digestibility and protein throughout the stockpiling period, particularly if legume leaves detach following frosts and during prolonged wet periods when nutrients can be leached from leaves.

Pasture forage testing can also support management decisions with leader-follower stocking, in which animals with high nutritional demands (such as lactating cows) consume the top horizon of a paddock then move on to the next paddock in a rotation sequence.

Animals with lower nutritional requirements, such as heifers or dry cows, follow and consume the remaining allotment of pasture down to no less than 3 inches before moving to graze what remains in the next paddock.

Pasture nutritive value for the follower herd is typically lower than what the leaders ingested, and forage testing can reveal the appropriateness of these canopy horizons for each animal class.

Role of NIRS

Near-infrared reflectance spectroscopy (NIRS) can be used on pasture samples for a quick assessment of current quality. Instrument calibration equations should be based on a broad range of freshly collected forage samples and geographical regions.

Calibration sets for pasture samples should represent grasses, legumes and volunteer species that are still palatable and eaten by grazing animals, such as dandelion and lambsquarters.

Other crops for extension of the grazing season or soil improvement should be included in NIRS pasture calibrations. Some of these include brassicas, small grains and mixes with peas, and cover crops.

In addition, stockpiled forage may have a great range of quality that should be represented in NIRS calibrations. If the database behind the NIRS calibration is robust, excellent predictions are available, resulting in profits from tracking quality during and over the pasture season. PD

Thomas Griggs is assistant professor of forage and grassland agronomy with the division of plant and soil sciences of West Virginia University.

Photo by PD staff.

patty laskowski-morren

Patty Laskowski Morren
Executive Director
NIRS Consortium

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