Forage quality evaluation has moved from rule of thumb to rule of rumen.
Mark Bowman, a ruminant nutritionist with Grand Valley Fortifiers in Cambridge, Ont., told the annual meeting of the Canadian Forage and Grassland Association in Guelph last November that forage testing has evolved significantly in the past three to five years, with more precise tools for livestock ration development.
The major innovation is different measures of digestion of forages in the rumen. Labs now use actual rumen fluid drawn from cows to determine rates of starch and fibre digestibility over varying times, as well as the rate of passage of fibre through the rumen.
The actual tests are supported by complex computational models which give nutritionists and farmers the data to work with.
Some of these new measures have led to the reintroduction of more grasses in dairy diets, versus alfalfa and the move to higher-forage versus grain diets.
Thirty years ago there were just rules of thumb, Bowman said. Forages were fed at about two per cent of body weight per day. For example, a 750-kg Holstein cow should get 15 kg.
“Today it’s a lot more complicated,” he said. Now rations are built from complex forage reports derived from comprehensive lab tests. There are formulation models that drive much of the decision-making on rations in the background.
Ration formulators have for years looked at neutral detergent fibre (NDF) as the standard measure of cell wall and cell contents — essentially how difficult it is for the cow to digest the forage.
“When we want to feed the cow, we are really feeding the rumen,” Bowman said. “As forage quality goes down, there’s only so much you can do to compensate.”
Today, there are numerous tests relating to NDF, such as dNDF (NDF digestibility). Labs also now test for uNDF (undigestible NDF). There are also other lab assays tested through NIRS (near-infrared spectroscopy). Then there are more complicated ration formulation models available for nutritionists to use. Acid detergent fibre (ADF) continues to be used.
Dave Taysom of Dairyland Laboratories Inc. told the CFGA meeting that uNDF is the undigested NDF residue after fermentation at a given length of time. It is accompanied by the time that the digestion is tested, such as uNDF240 for a test that has been run for 240 hours, or uNDF48, for a test that’s been run for 48 hours.
At one time the measure of digestibility was lignin. Even when low-lignin alfalfas were developed, they still looked similar to each other in lignin levels, but there is significant difference when one looks at the uNDF levels, Taysom said.
When Bowman gets a report back from the lab he first looks at the plant carbohydrate fractions, the cell wall and the cell contents. Protein is important economically, but “I can always feed more soybean meal,” if the protein level is low, he said.
Corn silage is also a major forage for dairy cows, providing energy and fibre. Testing has shown that fermenting corn silage before it’s fed will mean more digestible starch.
Bowman said that he likes corn silage to be fermented six months before it’s fed. If you only wait 30 days, then there will need to be more digestible starch put into the ration, likely through high-moisture corn.
That six-month storage is a challenge for many farms without more smaller silos.
Bowman said silage in a bag can work, “But if a farm uses tower silos, they’ll need two. Some producers just don’t have that.” The bottom line is that the higher the forage level in the ration, the cheaper and less risk the ration will be.
This article originally appeared in the 2018 Forage & Grassland Guide.