Supplementing Cows on Pasture Back »

While reading an article in “The Pioneer Woman” not long ago a statement caught my attention:

“…My dad always said that we are in the business of selling grass, not cattle…To most effectively “sell the grass” on our ranch, we are always evaluating what we think will work the best, “best” being a balance between what is most profitable and what is best for the land…”

There is a lot of wisdom in those lines, cows transform grass into saleable beef. However, in order to do this year after year, there needs to be an optimum balance between what is extracted from the land in animal products and what is left to allow for recuperation of the pasture. Just as in a bank account, the cow has a budget in his body with deposits and withdrawals; when one exceeds the other they can either build up or deplete her reserves. When turning cows on pasture one has to determine accurately what is going to become of that balance.

Energy is one of the most critical nutrients for grazing cows particularly at pasture turn out because it coincides with their greater needs to support the young calf. From a maximum energy requirement at peak milk production for a cow nursing a calf, nutrient demand decreases with days in milk. Requirements increase again in the last month of pregnancy due to the development of the fetus. This energy is used to fuel the body, and its balance (bank account example!) is the difference between the energy that is consumed with the feed, and that used by the animal for bodily functions. An absolute energy deficiency for example happens when there is good quality grass but not enough of it. This can be the result of not a very dense pasture, an excessive stocking rate, or both. Walking while grazing or between grazing paddocks and drinking water requires energy. Large-frame breeds have to carry heavier body weights during grazing. In addition, as the lean mass of the cow increases, so does its energy requirements for maintenance. This is because larger framed cows have also larger organs and these consume nearly 50% of maintenance energy. At the same time, energy expenditure due to muscle activity during grazing might also be significant depending on the terrain. The distance cattle walk daily varies both within days and between days on individual farms and is generally related to pasture availability and/or accessibility. Cows try to match their feed intake with their energy requirements. Supplementing cows on pasture is thus sometimes needed to achieve energy equilibrium and increase production and profit. The amount and type of supplement to use depends on each particular situation. In general, economics plays a major role in this decision. Although oftentimes we expect immediate responses in production, there is a need to evaluate the cost effectiveness of medium- and long-term supplementation programs on production and reproduction.

Group of Hereford cattle
Larger framed breeds have larger organs and require greater maintenance energy requirements.

Water Availability

Water availability determines how extensive each grazing bout can be, as pasture intake is related to water intake. It has been estimated that the optimum grazing distribution resembles a circle with the water source at its center and a radius of approximately half a mile, particularly during the summer. The advantage of maintaining this distance is that if individual cows are thirsty they will travel to the water alone and not with the whole group. As a result even if cows are not thirsty they will travel with the group reducing the time spent grazing. This distance changes depending on forage quality and availability and thus increases during a drought. The cow instinctively weighs the need for water against the energy obtained from harvesting the pasture and the energy used while walking to and from the water trough. Always remember that lush spring pastures are roughly 85 percent water and that in each 10 pounds of grass there are 1.5 pounds of water! Cattle have been described as having spatial memory that helps them find areas with high quality forage that they have grazed before. When introducing a group of animals to a new grazing area it is thus useful to include some that are already familiar with that environment. This enhances the use of the pasture, particularly during years or seasons of limited forage supply by reducing the learning curve of the recent arrivals.

Water availability diagram


Take Into Account Ambient Temperatures

Apparently season has an effect on the energy requirements of the cow independent of the ambient temperature. Cattle maintenance energy requirements are higher during the spring than during the fall. It has also been reported that as cow fatness increases, maintenance requirements increase during spring and summer but decrease during the fall and winter. Ambient temperatures have an effect whenever exceed the zone of thermal neutrality (usually around 41 to 68 F). Warmer days trigger mechanisms to dissipate heat (increased respiration and heart rate) and as a result, maintenance energy requirements also increase. As digestibility of the forage declines cattle have to ruminate more, which results in less time spent grazing. The time of the day when most grazing activities occur is determined by climate and in particular by ambient temperatures. With ambient temperatures at or close the zone of thermal neutrality, grazing occurs mostly during the day. During warm days, there’s a shift in grazing patterns with more early morning, late afternoon, and night grazing. Night grazing is performed in the area close to where it ended in the evening as there is little directional grazing at night. If rotational grazing is used during warm weather, animals should be assigned high quality paddocks particularly in the evening to maximize the daily grazing hours, improve pasture utilization, and optimize performance.

Good Pasture Availability

Figure 1 shows an example of an abundant grass pasture in South Dakota grasslands. This pasture provides enough energy for grazing cow-calf pairs, unless stocking rate is inadequate or grazing is prolonged beyond what is reasonable. If the latter is the situation, the energy of the forage might be adequate, but the deficiency results from the animals not harvesting enough. The optimal suggestion is to allow cows to get full while leaving almost 50% of the pasture. In optimum weather growing conditions oftentimes one could get away with almost grazing 2/3 of the pasture without hampering its regrowth. Keep always in mind though that the negative effects of overgrazing are associated with exposing more surface terrain to the sun which will increase water losses.

South Dakota grasslands
Figure 1. Abundant pasture in South Dakota grasslands.

Supplementation strategy: The dense grass canopy protects the soil surface from water losses by evapotranspiration which encourages fast regrowth. This pasture is also high in water content, which will contribute to the daily requirements of the cows, reducing the daily trips to the location of the drinking water. This results in less “dead time” walking, more energy harvested from the pasture, and less energy wasted in exercise. Protein is usually enough in these pastures to cover the requirement of the cows. There is a need however of supplementing with white salt with trace minerals free choice. A primarily salt block like the one in Figure 4 will likely work well; there are also commercial primarily salt blocks that contain sulfur as the trace mineral. In cows in early lactation the mineral/vitamin block in Figure 3 could give better results particularly right before breeding.

Supplementing with round bales in this situation is not necessary and in fact it can be counterproductive to the pasture. If the bales are added to try to solve an over-stocking problem, the cows will eat the bales only after they have overgrazed and likely compromised the regrowth of the pasture. Research has shown that as forage availability decreased to 500 lbs. per acre there was a four-fold increase in rate of consumption, and a two-fold increase in time spent grazing. At higher stocking rates animals are pushed to graze closer to the ground. Quality, and thus energy supplied by the pasture, decrease while energy expenditure due to activity becomes less important. Increasing stocking rates may also induce more dominant/submissive behavior, which can modify feed intake.

Limited Pasture Availability

Figure 2 shows a situation that is not that uncommon, which is the combination of sparse pasture with uneven terrain. The green color is an indication the pasture is in active photosynthesis and growth. The digestibility is likely high and as a result the energy it supplies, however the pasture mass available to the cows is limiting. In this scenario the animals will spend additional energy while walking to harvest less than optimum amounts of forage. This is further complicated by the slope of the terrain which requires additional energy when compared to flat terrain.

Cow on sparse pasture, uneven terrain
Figure 2. Sparse pasture and uneven terrain.

Supplementation Strategy

Under these conditions and depending on the stocking rate it is likely that there is a need to supplement with forage (round bales) and likely some grain. In addition, the animals will also benefit from the supplementation with one of the multi-nutrient blocks (Figure 5 and Figure 6). The choice will depend on the quality of the hay bales used, with more fibrous hay the multi-nutritional block in Figure 6 might provide a better response because of its higher concentration in protein. Note however that half 0f that protein (18.5%) comes from non-protein nitrogen, likely urea.

Guaranteed Analysis: Mineral/vitamin block
Figure 3. Mineral/vitamin block.

Choosing the Right Supplement

When supplementing grazing cattle, the objective is to balance nutrient deficiencies and to do so in an efficient and profitable manner. From everything addressed above it is clearly important to take into consideration the feeds and the season. For example, feeds that generate excessive heat of fermentation (fibrous forages) are inappropriate as a supplement during hot summer days but are necessary during the winter. On the other hand, supplementing with concentrates during hot weather, although they increase the nutrient density of the diet and produce less heat of fermentation, may result in cases of sub-clinical to clinical acidosis and may reduce the rate of fiber digestion. Energy is probably one of the first limiting nutrients under grazing conditions. Energy is mostly derived from pastures and forages. Energy deficiency can be primary (resultant of low forage fiber digestibility) or secondary to a suboptimal level of other nutrients such as protein. In the case of a secondary energy deficiency, supplementing with the nutrient which is deficient (e.g. nitrogen) will result in increased digestible organic matter being utilized by the rumen microbes. In instances where sparse and/or overly mature pastures are grazed, you often will see a combined energy/protein deficiency rather than that of a single nutrient. Under these conditions using multi-nutrient blocks such as those in Figure 5 and Figure 6 will provide a better animal response, particularly if accompanied with some bales. Minerals and vitamins are also important nutrients to take into consideration. Of the macro minerals (those present in greater concentration in the diet) phosphorus, sodium, and chlorine are the most likely to be deficient under range conditions. Magnesium deficiency can present itself as grass tetany, which occurs during pasture growth during spring or fall. Deficiencies should always be a concern when animals graze pastures that have been under stress (e.g. drought) over a prolonged period of time.

Guaranteed Analysis: Primary salt block
Figure 4. Primary salt block.

Cobalt, copper, iodine, manganese, selenium, and zinc of the micro or trace minerals are more than likely the ones that will be deficient under range conditions. However, in certain areas selenium may be present in concentrations that are potentially toxic. This can occur via selenium accumulator plants or via traditional native or tame grasses. It is also possible that secondary deficiencies can be caused by mineral antagonisms. This is particularly true for copper. Iron, molybdenum, and sulfur are all potent antagonists against copper absorption. In pastures that are mature and dry, beta-carotenes, which are precursors of vitamin A, are often deficient. Plants pro-vitamin content can be assessed fairly accurately by looking at their color. Green indicates the presence of beta-carotenes, whereas a yellowish color indicates their depletion and/or oxidation. Under these circumstances it is also more than likely than vitamin E can also be deficient.

Guaranteed Analysis: Multi-nutrient block
Figure 5. Multi-nutrient block.

Guaranteed Analysis: Multi-nutrient block
Figure 6. Multi-nutrient block.

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