July 10, 2013
One of the ways grazing management pays is when you use it to set up much better nutritional content in forages.
For example, consider this chart summarizing data from research by Jason Rowntree at Michigan State University.
Rowntree says this data shows the mean, or average, of a full season's worth of forage samples. Those samples were taken from the top third of the canopy, middle third of the canopy and bottom third, as labeled. Also, all the samples were from mixed swards of forage and not from individual plants. It also included a range of plant maturities.
CP is crude protein and IVDMD means In Vitro Dry Matter Digestibility.
Just the top: The data on forage quality can be confusing because most of what's available is whole-plant analysis. That's misleading unless you plan to make your cattle eat the whole plant.
This helps show that developing and grazing fairly mature stands of forage, meaning relatively "tall" forage, and only asking the cattle to graze the top part of the plant will provide the highest level of nutrition.
In fact you may contrast that with young forage, or with forage which has been grazed and not allowed ample recovery time, which typically has overall high protein content and overall low energy content.
Jim Gerrish says of the Michigan data: "The data and the energy-to-protein ratio is consistent with what I saw in every set of stratified samples I ever took -- which was in the thousands." Gerrish now owns American GrazingLands Services in Idaho. For many years he was the research leader at the Forage Systems Research Center at Linneus, Missouri.
Gerrish adds, "If you do analysis based on individual species and not a mixed sward, the data will look essentially the same, just with more variation across height for some species compared to others."
"We also did hundreds of individual species analyses," he adds. "Thousands of other studies over the last 100-plus years shows the same thing."
Gerrish explains IVDMD has been used as an estimate of energy content of forage for around 60 years.
"There are some who say it doesn't mean anything but you can generally predict cattle rate of gain within 90% accuracy using IVDMD, as long as forage intake is not restricted by bite size," he says.
The ratio of IVDMD to CP also tells us about expected animal performance.
Gerrish says going back to his days in graduate school, the target IVDMD:CP ratio for a healthy, functioning rumen was anywhere between 4:1 and 6:1.
For higher-production animals, the target was nearer to 4:1. An example would be IVDMD at 64% divided by CP at 16% = 4:1.
For animals at maintenance, the target was 6:1 such as IVDMD at 48% divided by CP at 8% = 6:1.
When the ratio drops below 4:1, Gerrish says, you need to supplement a digestible substrate such as corn, grass hay or straw. When the ratio rises above 6:1 you need to supplement with protein, such as limited access annual ryegrass pasture, alfalfa hay, DDG or cottonseed products.
Applying this information about IVDMD-to-CP ratio and expected animal performance, the top third of the plant would be expected to give the highest animal performance, the middle third moderate performance, and the bottom third is maintenance feed, Gerrish says.
He adds that this information has been the prevailing theory in grazing science for many years and was the foundation for the development of leader-follower grazing sequences developed in the 1950s and 1960s.
"It is why top grazing with finishing animals and cleaning up with cows works so well," he says. "Dr. Roy Blaser at Virginia Tech and Dr. Jerry Matches at University of Missouri pioneered these finishing systems in the 1950s and 60s. Leader-follower was also widely promoted as a dairy grazing strategy in the same time period."
Rowntree says he looks especially at NDF (neutral detergent fiber) in developing management and he often grazes for less mature forage than some people do these days.
"For our system the number I focus on is NDF," Rowntree says. "I try to keep it real simple. If I can keep my NDF in the 40's I can get the gains I need in the grassfed side and be very profitable. Intake is the driver for profitable gains. An animal can eat around 1.1% of NDF daily. I want to get 3% [body weight] intake in my growing steers.
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