Grazing plans must balance supply and demand
September 28, 2008
Feed available from pasture for winter grazing is the inevitable result of spring/summer growing conditions and stocking rate decisions made at the beginning of the grazing season and adjustments made subsequently. In the Tri-State region, persistent drought in western North Dakota contrasts dramatically with the adequate to abundant moisture received in adjoining areas. While spring plans for grazing anticipate possible growing conditions and must incorporate flexibility to adjust, particularly to precipitation deficits, grazing resources available for fall and winter grazing can be assessed with considerable accuracy. The analog to unknown precipitation during the growing season is the uncertainty of snow cover and/or blizzard conditions during the winter. Both possibilities demand flexibility and adequate contingency planning.
Accurate assessment of standing vegetation that can be grazed will be extremely valuable in the current environment of high supplemental feed costs. The most certain procedure for making such an assessment would be to clip and weigh representative samples from the pasture or pastures being evaluated. The equipment and time required for such an assessment is a bargain when compared to the value of the information developed. Clipping a known area, drying and then weighing the samples allow calculation of the pounds per acre of vegetation available. Clipping samples into paper bags and leaving the bags on a vehicle dashboard with the doors and windows closed for a couple days will provide adequate drying so that calculations can approach “dry matter basis.”
Estimates of intake, for a dry cow for instance, can be made based on body weight. Consumption of mature range vegetation by a dry, pregnant cow will rarely exceed 2.5 percent of body weight during the first two trimesters of gestation. A 1,250 pound cow would be expected to consume about 30 pounds of dry matter per day. Comparing assessments of feed available to feed required provides a dependable estimate of the number of days grazing that will provide adequate feed for a known number of animals.
A frequent question concerning winter grazing is, “can more material be removed from dormant vegetation, than from growing plants, without long term damage?” For example, if I can safely remove 50 percent of total biomass during the growing season, can I graze to remove 75 percent from dormant vegetation? The standard, conservative response is that the “take half/leave half” rule of thumb applies to winter as well as summer grazing. While plant biomass will not contribute to immediate regrowth during the winter, grazing to maintain adequate residue provides at least two benefits.
Many plants, especially warm-season grasses, store nutrient reserves in above-ground stem bases, as well as below ground structures. Excessive grazing can remove portions of these reserves. The short term gain in feed harvested will result in slower spring recovery and growth. The consequence could be lower feed supplies and weakened stands during the following growing season.
The second benefit of adequate residue during winter is snow capture. Moisture supplied to vegetation during winter and spring snow melt can be extremely important to early season growth.
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Another assessment important to rationing winter feed is the condition of animals going into the winter. Body condition scores provide an important index of feed necessary to maintain adequate animal performance. While pine trees don’t occur on every ranch, the impact of body condition on consumption of pine needles illustrates the value of accurately assessing body condition scores. Late term abortion has been associated with pine needle consumption for many years. Ponderosa pine, common to rangelands in our area, has been clearly identified as accumulating potentially harmful levels of isocupressic acid (ICA), the compound responsible for late term abortions. When ICA is ingested, blood flow to the placenta is reduced and nutrient supply to the fetus is compromised. Effects are dose dependent and cumulative. Abortions result when large enough quantities of needles are consumed over a long enough period of time. Where ponderosa pine occurs in rangeland, it is often in broken terrain, ideal for providing protection during late winter and early spring.
Research conducted by scientists at the USDA Poisonous Plant Laboratory demonstrates the impact of body condition on pine needle consumption. Cows in low body condition (less than 4.0) were compared to those in high body condition (7.5) in a series of five experiments. While pine needle consumption was variable, in every experiment, cows in low body condition consumed more pine needles than cows in high condition. All cows consumed more pine needles in cold or snowy weather, but the increase was greater for cows with low condition. The lone exception to these results was when cows were fed a low protein diet.
Initially, cows in low condition consumed more pine needles, but this difference disappeared after about three days. Scientists concluded that dietary protein/energy ratio may influence pine needle intake. Use of pastures where pine needle consumption is a risk should be minimized during late gestation and be avoided altogether with cows in low body condition.
All grazing plans must balance supply and demand. Using simple tools such as body condition scoring and pasture biomass inventory to precisely evaluate the supply and demand can guide grazing decisions which can minimize feed costs by maximizing the recovery of feed from pastures without sacrificing future productivity. Careful planning, which includes flexibility and provision for contingencies will minimize unexpected or undesirable outcomes.
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