When is spring pasture too good?
This item has been supplied by a forage marketer and has not been edited, verified or endorsed by Hay & Forage Grower.
How can any feed be too good, anyway? With the cost of feed the largest expense on many operations (especially dairy farms), spring green-up is (mostly) a time for celebration – things are warming up, days get longer, we get ourselves and the cows back outside, and with that, we can get ahead on issues like manure, pen maintenance, calf health, and labor demands. Putting the cows out on pasture greatly reduces the inputs, but care is needed to boost animal health and really maximize profitability. As perennial pastures break dormancy in spring, the new flush is marked by rapid growth, leading to fast nutrient uptake and, as a result, distinct nutritional composition and moisture content from what we learn to expect of pasture species throughout most of the year. Rapidly growing plants are high in moisture and nutrients can get diluted. With still-cold nights, growth can stall while sugars continue to accumulate. This results in more concentrated sugars and sweeter plants (of particular concern to horses, since horses don’t digest a type of non-structural carbohydrate known as fructans well and can suffer from high blood sugar and laminitis if they don’t get enough effective fiber to balance this out). |
Most of us think of high quality feeds rich in protein and nutrients and low in lignin as the key to maximizing dry matter intake. It turns out that there is a certain amount of digestible fiber and energy required, however, to maintain rumen microbes and healthy rumen function. That first spring flush is like candy – highly tempting to the cows, but not a balanced diet. High in moisture and soluble carbohydrates, it can fill up the rumen before the cow gets enough to meet her energy demands. Physical fill can easily be the limiting factor here. The rumen of a 1400-lb cow needs at least 138 lbs of fresh grass to meet her energy requirements, but fills up with 100-125 pounds of fresh matter. This may go without saying, but higher levels of milk production mean more dry matter is needed. Cows producing over 50 lbs of 4 percent fat-corrected milk need more energy intake from concentrate or silage sources.
These grasses are high in protein but have only a moderate energy content, which can lead to protein deamination if the feed becomes a principal part of the diet – ammonia production which can then get into the blood via the rumen epithelium (a membrane that separates the rumen from the ruminal veins and main bloodstream), then is converted to urea by the liver and excreted in urine. Excess protein ultimately detracts from reproductive performance. Before excess protein becomes acceptable, the ration will have to supply adequate or above adequate energy levels. If cows end up with too much ammonia in their systems, they can’t get enough oxygen, with may manifest as panting.
Even the highest quality, most balanced pastures don’t provide cows with adequate energy to meet their genetic potential for gain or milk production, just because it’s so hard for them to consume enough to get the energy and nutrient content they need. But at certain points during the year, supplementation of pastures with grain or hay is more necessary, like early spring or during the summer heat.
The tricky part is finding the sweet spot of supplementation at each point in the season, and balancing cow needs for the right quality range with pasture needs. Too much supplemented feed and the purpose of the grazing program is defeated. Too little, and the cows might overgraze.
A central pillar of cow nutrition, the rumen lives in a delicate equilibrium. Rumen health depends on its production of microbial protein, and digestible fiber is one of the essential food sources of rumen microbes. John Hibma wrote in Farming that the catch-22 here is that “effective fiber diminishes with highly digestible forages. Lowering dietary effective fiber will have a deleterious effect on rumen microbial synthesis [translation: free protein made in the rumen, fueled by forages the cow has eaten]. Just when you think the pastures are the best they’ve ever been, you see a drop in butterfat and milk production because the rumen is having problems. Maximizing and sustaining high milk production on pastures is both an art and a science.”
David Hunsberger of King’s AgriSeeds suggests, “something I promote heavily, if using a TMR is a possibility, is to treat grass as equivalent to a TMR. Feed to refusal, then just subtract the cow numbers to match grass intake. It’s very pragmatic and almost removes the stresses.” It’s a way to manage different feedstuffs, but also allows more flexibility to adjust the proportion of feeds depending on which you are rationing and which you want to promote. It goes a long way to preventing the early spring dangers of bloat and grass tetany because you can place priority on the TMR, and make sure that effective fiber and magnesium level are balanced. Then pasture grass becomes supplemental, but these roles shift as season progresses, depending on pasture quality and availability. Many successful pasture based systems use a combination of high quality pasture and stored or purchased feedstuffs in attempt to balance the digestibility of intake as a whole.
If feed is too digestible and NDF gets below 35%, rumen pH can drop below 5.8. Maintaining an ideal rumen pH of 5.8-6.2 can be a challenge with high quality grass-legume pasture.
Additional possible approaches include adding 1-2 lbs of straw, 5 lbs of baled hay, or 10 lbs (dry matter basis) of silage, which can provide enough effective fiber to slow down rapidly fermentable fiber and manage rumen turnover. Move cows every 12-24 hours to maximize intake, and control amounts of supplemental feeds closely. Effective NDF contributes to rumination and maintains rumen function.
As with many biological systems, optimal digestibility lives within a range. Go above or below, and you upset the cows’ biological systems and compromise performance. A few signs that digestibility might be below optimal:
Other dangers hiding in new spring growth…
Spring grasses breaking dormancy are a common cause of tetany, but autumn tetany (though relatively rare) can also occur with lush fall regrowth. It also becomes a concern when grazing cool season grasses during cool, cloudy, rainy weather, or when cool weather is followed by a warm period.
Despite challenges, grazing can certainly pay off. It often returns higher net farm income per cow, in terms of comparative dollar advantage. Although you get less overall production per cow, vet and machinery costs are lower, as is investment per cow and culling rates. Grazing systems work when you can meet the nutrient needs of animals and avoid large investments in facilities and equipment.
Submitted by: Genevieve Slocum, King’s AgriSeeds Inc.