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Bred cows need additional energy and protein

Straw is a major ingredient in beef cow rations this year due to the shortage and high cost of hay in many parts of the country, particularly Western Canada. With the cold of winter now upon us and herds entering the third stage of gestation it’s important to understand the special considerations when feeding straw-based diets.

A key point to remember is that dry matter (DM) intake is relative to the digestibility of the forage. Cows are physically able to eat more of high-quality forage that is easily digested than they will be able to eat of a poor-quality forage that takes a longer time to pass through the digestive system, explains John McKinnon, beef chair at the University of Saskatchewan.

A mature cow will typically consume 2.0 to 2.5 per cent of her body weight on a DM basis. A 1,300-pound cow, for example, will consume 26 to 33 pounds DM per day. That sways to the high side when medium-to high-quality forage comprises the ration and can even reach 3.0 per cent if cows are given free-choice access to hay that is very good quality. However, a ration based on cereal straw or low-quality hay will restrict DM intake to 1.0 to 2.0 per cent of body weight due to poor digestibility and, in some cases, poor palatability.

Given free access to barley straw only, a cow might be able to eat enough to “feel full,” however, the nutrient content won’t be sufficient to meet her daily nutrient requirements even under average weather conditions.

“The fact is that this byproduct of grain production is deficient in all essential nutrients including energy, protein, calcium and vitamin A,” McKinnon explains. If she is physically unable to consume adequate forage to obtain her basic requirements, you will have to adjust the ration accordingly by providing supplemental feed.

The ration has to supply a combination of energy, protein, major minerals (i.e. calcium and phosphorus), trace minerals (i.e. copper, zinc and manganese), and vitamins (i.e. vitamin A, D, E), for the cow’s maintenance and weight gain as well as for fetal growth and development.

McKinnon reminds producers that failure to supplement straw-based diets with adequate energy, protein, vitamins and minerals will result in weight loss and poor body condition, which will have negative repercussions on the cow’s ability to rebreed.

Straw-based rations pose additional concerns when the weather turns cold because of the risk of impaction. This condition can occur when cows increase feed consumption to increase energy intake to maintain their body temperature. Over-consumption of a poorly digested feed, such as straw, can lead to excessive buildup of feed in the rumen and in the other parts of the cow’s digestive tract, particularly the abomasum. The undigested straw can’t pass through to the intestines to complete the digestive process. Affected animals will go off feed. Total impaction can result in death, while partial impaction leads to reduced feed consumption and weight loss. A veterinarian should be

consulted if you suspect impaction problems when feeding straw-based rations.

“A reliable water supply and feeding extra grain will help prevent impaction when the weather turns cold,” he says. “In this situation, supplemental grain provides energy to rumen bacteria to help ferment the straw in the rumen and prevent buildup of undigested residues in the rumen and abomasum.”

Understanding energy and protein

Energy keeps animals alive and productive, McKinnon explains. It is the nutrient required in the largest amount and the most costly to supply.

Factors affecting the amount of energy required by an animal include its age, body weight and condition, hair and hide thickness, stage of pregnancy and weather conditions. Ideally, cows coming off pasture in the fall should be in moderate to good condition with a body condition score (BCS) of 3 on a 5-point scale. The goal of the nutrition program is to maintain these animals in this condition throughout the winter, calving and breeding. Cows coming off pasture carrying excess condition (BCS greater than 3.5) will be able to use their body fat stores as an energy supply through the winter, he adds, but try to minimize weight loss during the last six to eight weeks prior to calving and through breeding.

Most feed laboratories still provide producers with an estimate of the energy content of a feed expressed as total digestible nutrients (TDN). As the name implies, TDN is the percentage of the nutrients in a feed that are digestible. As such it is a general measure of the nutritive value of a feed, explains McKinnon.

Another measure of feed energy is digestible energy (DE) — measured in megacalories (Mcal) per kilogram of DM. Typically, DE is calculated from the TDN value of a feed and thus the two measures are directly correlated. That is, feed ingredients with higher TDN will have a higher DE content.

A final measure of feed energy that producers can encounter from a feed test laboratory is net energy (NE). According to McKinnon, NE is the amount of energy available to the animal after accounting for losses in digestion, body metabolism and heat production and is the energy content of a feed that can actually be used for maintenance and productive purposes.

Protein is the second most important nutrient in terms of the quantity required. It provides essential building blocks (amino acids) for growth, pregnancy and lactation. Age, body weight, growth rate, stage of pregnancy and lactation are all factors affecting protein requirements.

According to the National Research Council (NRC, 2000), a 1,300-pound wintering beef cow in her second trimester with a BCS of 3.0 requires about 13.5 pounds of TDN and 1.5 pounds of protein per day. Providing 26 pounds (2.0 per cent of body weight) of good-quality brome hay (DM basis) will meet her daily energy and protein requirements under typical winter conditions. Remember though, as this cow moves into her last trimester of pregnancy, energy and protein requirements will increase by as much as 23 and 33 per cent (NRC, 2000), respectively and energy requirements can be further increased by severe cold stress, he adds.

The challenge

A good rule of thumb when formulating rations based on cereal straw is to limit the straw portion to 1.25 to 1.5 per cent of the cow’s body weight on a DM basis. In the case of our example cow, this would mean 16 to 19.5 pounds of straw DM a day or 18.5 to 22.0 pounds as fed, McKinnon says.

To meet her requirements, she will also need to be supplemented with both an energy and protein source. Typically, this will involve feeding a cereal grain or its equivalent (i. e. fortified grain screening pellets). It’s not unusual in the second trimester to have to feed eight to 10 pounds of barley grain when straw makes up the forage component of the ration.

McKinnon points out that if your mainstay ration for the winter is based on straw and grain, you will have to feed relatively high levels of grain or an equivalent feed source to meet energy requirements during cold weather and the third trimester of pregnancy. For example, our 1,300-pound cow consuming 18.5 pounds of straw would require approximately 12 pounds of barley grain under normal winter conditions (i.e. -15with minimal wind) during her third trimester to meet her energy needs and for normal fetal development.

Cold stress can further increase the animal’s energy needs and increase the need for grain supplementation. Depending on the situation, the grain portion of the ration may need to be increased by one to two pounds for every five degrees the daytime temperature drops below -20C. This is particularly critical if the severe cold is expected to last for a prolonged period.

“Feeding that much grain to beef cows can be a challenge in itself! Cereal grains such as barley, wheat and corn are high-starch feed grains that digest rapidly in the rumen. This rapid digestion, particularly in cows unaccustomed to grain feeding, can lead to digestive upsets such as bloat, grain overload and acidosis,” McKinnon explains. He recommends adapting cows in a series of two to three steps and once adapted, feeding no more than eight to 10 pounds of grain at one feeding. Oat grain is slightly lower in energy content than the above-mentioned cereal grains, however, it is not as difficult to feed because it is digested slower in the rumen due to its fibrous hull.

Straw and grain contain adequate phosphorus to meet a cow’s requirements, but they are low in calcium. Limestone at 1.5 ounces per day during the second trimester, bumped up to 2.0 ounces per day during the third trimester, is an inexpensive way to meet calcium requirements in cereal straw-grain-based rations.

McKinnon wrapped up his presentation by reminding producers that optimum reproductive efficiency is based on a sound nutrition program that matches the quantity and quality of the cow’s ration to her nutrient requirements for maintenance and productive functions particularly pregnancy. These requirements change with stage of pregnancy and are impacted by the cow’s body condition and environment. Producers must adapt their nutritional management to meet these changing needs, particularly when faced with a feed shortage and the need to rely on poor-quality forages to get them through the winter.

For assistance with developing straw-based feeding programs, producers are encouraged to consult a qualified nutritionist.

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