Murray Snowdon, Livestock Nutritionist
At a time when recycling and waste reduction are foremost in so many people's minds, it is interesting to note that cattle have been used as recyclers since man first domesticated animals and continue to play an important role in making use of unwanted food wastes, often referred to as byproduct or opportunity feeds.
Because many byproduct feeds are wet or bulky, their use is often limited to the geographical area in which they are produced. Cull potatoes, one of the most readily available byproduct feeds in New Brunswick present a true opportunity for livestock feeders in this grain deficient region.
Culls are rejected for a number of reasons; size, colour, shape and disease are all factors in culling. Growing season, potato market value, and time of year all play major roles in cull availability, cost and quality. As many as 135 million tonnes of culls are available each year in New Brunswick.
Handling and Storage
Because a 10 tonne truckload of potatoes contains almost 8 tonnes of water there are severe limits on the distance which potatoes can be hauled economically.
Since potatoes can freeze, an insulated storage would be an ideal way to store them in the winter months, but the cost of erecting a building solely for this purpose is not justified. Storing cull potatoes, especially washed ones, presents problems in warm weather as well. These washed potatoes are particularly prone to spoilage. Obviously, rapid feedout is the best policy any time of year.
Producers should check with their supplier in order to avoid taking delivery of culls that contain a lot of rocks. These are hard on equipment and removing them from feedbunks is a time-consuming job.
Whole potatoes can be ensiled by putting them in layers in a horizontal silo with well wilted hay crop silage. Since cull potatoes are not always available when silos are being filled, this option is often not possible.
A mixture of 3 parts chopped potatoes to 1 part chopped hay can also be ensiled. The costs associated with this approach have made its use very limited.
The wet bulky nature of potatoes makes them best suited to a mechanized feeding situation where time and labour involved in handling are minimized. Many producers who attempt to feed potatoes in situations that require a lot of hand labour soon become discouraged and return to more traditional feeds.
Cull potatoes' main contribution to a livestock diet is energy; their high starch content puts them on par with feed grains in terms of energy content when considered on an equal dry matter basis.
Of course, moisture content must be taken into account when buying or feeding with 4.5 kilograms of potatoes required to replace one kilogram of corn or barley.
Since raw potato starch is quite resistant to digestion, feeding large amounts of potatoes will result in excessive starch bypassing the rumen. If this undigested starch reaches the lower intestinal tract, digestive upsets are likely. The recommended upper limits for potato feeding are in part a reflection of this concern.
Because of their very low fibre content, potatoes should not be considered a forage substitute but rather should be thought of as a high moisture grain. Potatoes are quite low in protein content and when fed in high amounts without protein supplementation will not give good animal performance or feed efficiency. The higher protein requirements (as a percentage of diet) of lighter cattle make protein supplementation especially important for light cattle fed potato diets. Typical potato nutrient content is given in Table 1.
Potatoes should be introduced to rations gradually, particularly in the case of cattle which have been recently shipped. Feeding high levels of a wet, starchy feedstuff such as potatoes places an additional stress on these animals and may lead to health problems.
Although some adaptation is required, potatoes are quite palatable and are readily consumed by livestock. In research trials, dairy cows have consumed over 45 kg of cull potatoes per day.
Despite high levels of voluntary consumption, animal performance usually drops when potatoes make up much more than 30% of the diet dry matter; for most feeding situations it is best to restrict potato intake to levels lower than this.
Potatoes are an excellent energy source for ruminant livestock (cattle and sheep) but the presence of anti-nutritional factors, as well as the difficulty in digesting potato starch make raw potatoes low in feed value for pigs. Cooked potatoes make a good energy source for pigs but feeding raw cull potatoes directly to cattle usually makes more economic sense.
Because potatoes are 75-80% water, care must be taken to avoid combining them with other wet feeds to give a diet with greater than 70% moisture content; wet rations often lower feed intake and daily gains particularly in cold weather.
The wet manure that high moisture diets often cause makes it difficult and costly to keep animals dry and comfortable and is another reason for reduced performance on wet rations.
The digestive problems sometimes blamed on the high moisture content of potatoes are in fact related to their very low fibre content. When high levels of potato are fed, a source of fibre (roughage) is necessary in order to maintain a normally functioning and healthy rumen.
In the case of dairy cows, failure to provide enough roughage will cause a drop in milk fat test in addition to the possible digestive upsets associated with low fibre diets. The standard recommendations for minimum fibre (forage) still apply but should be given special attention when rations contain high levels of potatoes.
As little as 1 kg of hay per head per day will provide sufficient fibre for short-keep feeder cattle but cattle that will be on feed for greater than 60 days should receive more than this minimum.
Potato sprouts and potatoes that have turned green from exposure to the sun contain elevated levels of toxic alkaloids. Although a moderate number of sprouted or green potatoes are not cause for alarm, large amounts of this type of potato should be avoided.
The size and shape of potatoes make choking a possibility in potato-fed animals. Although animal losses to choking are relatively rare, the worry associated with the risk is an aggravation to some producers.
If feed bunks are designed so that the animals' heads are down while eating, the risk of choking is minimized. Having adequate bunk space helps to reduce competition for the potatoes and will also reduce choking. Small potatoes, especially if frozen are the most likely to cause choking and should be fed with caution. Chopping potatoes is the only way of eliminating the risk of choking but the payback on this practice is questionable.
Some farmers restrict access to water or withhold water altogether from cattle being fed high levels of potatoes. The theory is that by reducing water intake the dry matter content of the feed in the digestive tract is reduced, the manure is drier and the cattle perform better. Despite the fact that this method appears to work for some farmers, the practice is not recommended because of the risk of dehydration and reduced animal performance.
Best results will be obtained when a long term, regular supply of potatoes can be obtained. A short term or sporadic supply is of questionable value to a feeding program.
Although exact feeding recommendations will change with every situation, a few guidelines apply generally. Table 2 provides feeding guidelines for various classes of cattle. Most of these levels could be safely exceeded but the likely drop in animal performance and feed conversion make the economics of doing so questionable.
Protein Supplementation For Potato Rations
Potatoes make a significant contribution of energy to a diet, but a relatively small contribution of protein. Failure to provide adequate protein supplementation accounts for the disappointing results that some farmers have experienced when feeding potatoes.
If potatoes are fed as a total or partial replacement for a grain mixture Table 3 provides some guidance on substitution levels.
For example, if potatoes are used to replace a 14% protein grain mix, each unit of the grain mix removed represents a mixture of energy, protein, minerals and vitamins. Potatoes readily replace the energy and partially replace the protein but make a very small contribution to the minerals and vitamins in the ration.
Mineral and Vitamin Supplementation of Potato Rations
Since potatoes are quite low in minerals (calcium and magnesium in particular), and vitamins, ration adjustments will be necessary for livestock fed a lot of potatoes. In some cases, the required minerals and vitamins will be present in the commercial protein supplement or can be incorporated into a custom protein supplement. In either of these situations, free-choice salt should still be provided since salt requirements may increase on a wet diet.
Preference should always be given to adding supplementary minerals and vitamins directly to the diet, either in the grain mix or in the feed bunk. Cattle cannot be relied on to eat the required amount of free choice mineral; over or under consumption is common when this method is used. Free-choice minerals are best considered a supplementary safeguard rather than a vital part of the diet.
In some cases free-choice feeding is the only practical way of providing supplementary minerals. In this situation, choose a mineral that is as complete as possible. Virtually all minerals will contain both calcium and phosphorus in various amounts. One that contains magnesium and selenium should be considered a plus. High vitamin levels are also an asset if a regular vitamin injection program is not being followed. In most situations a mineral mix which contains salt is also desirable.
If minerals are being topdressed in the feedbunk, between 50 and 100 grams per head per day is a reasonable amount. The exact amount required will vary depending on the other feeds in the diet and the mineral supplement being used.
Remember that trace-mineralized (TM) salt is not the equivalent of a commercial mineral. As the name implies, TM salt contains trace minerals (copper zinc, etc.), but it does not contain the macro-minerals calcium, phosphorus, and magnesium and cannot be considered a complete mineral supplement.
The factors that need consideration when contemplating the use of potatoes in a ration are similar to those for any wet byproduct. Purchase cost, handling cost and consistent supply are all major factors.
Some careful thought needs to be put into the total cost of putting potatoes in the feedbunk. Depending on location and the storage and handling systems involved, costs to put this feed in front of the cattle may be considerable. The price paid for the potatoes should allow for the additional expenses expected in hauling, storing and feeding.
Since potatoes are low in protein and high in energy, you should look at how potatoes compliment the other feeds in the ration. If all of the farm feeds available are low in protein then potatoes may not be the ideal choice for the ration.
Potatoes are a high energy feed that must compete with more traditional feed grains. When potatoes are fed at moderate levels, animal performance should be similar to cattle fed equivalent amounts of dry grains. The cost of potatoes and the labour involved in handling them will be major factors in determining their feasibility in a ration.
TABLE 1 TYPICAL NUTRIENT LEVELS IN CULL POTATOES
Nutrient Level, %
|As Fed||Dry Basis|
TABLE 2 SUGGESTED MAXIMUM LEVELS FOR POTATO FEEDING
|Livestock Category||Animal Weight||
|Under 300 kg
Over 300 kg
|Dairy & Beef Cows||
|Growing-Finishing Beef Cattle||Under 300 kg
Over 400 kg
TABLE 3 SUGGESTED POTATO-GRAIN SUBSTITUTE LEVELS
|Potatoes Used to Replace||For each kg of grain mix removed substitute:|
|12% protein grain mix||3.5 kg potatoes, .2 kg protein supplement*|
|14% protein grain mix||3.4 kg potatoes, .25 kg protein supplement|
|16% protein grain mix||3.3 kg potatoes, .35 kg protein supplement|
|18% protein grain mix||3.2 kg potatoes, .40 kg protein supplement|