2 - Understand livestock nutritional requirements

Here we discuss the nutrient requirements of cattle and how nutrients are made available to your livestock. The productivity of a cattle enterprise in an arid environment is largely determined by the quality and quantity of feed available to the cattle. Without the convenience and ability of being able to improve pastures and tailor feed it is important to be aware of the nutritive values of pastures on your property and the requirements of your cattle at any given life stage. There are also a range of factors that will influence the feed intake of your cattle that need to be understood.

Knowledge of what is required by your livestock at any stage can help you plan livestock movements and determine carrying capacity and stocking rate (these terms will be discussed in detail later in this module). This serves to ensure the highest level of production while maintaining the condition of your pastures and paddocks. Pastures with greater diversity of plant species not only represents a healthy biological system but also provides better nutrition for your cattle, giving them a range of fodder choices with different nutritional compositions and therefore more balanced diet.

There are a number of factors that will influence the nutritional requirements of your cattle including class, life/breeding stage, environment and stock preference. Also, the presence of minerals and nutrients in feed does not always mean that they will be available to your stock. For example, a high level of one mineral can prevent the absorption of another.

Livestock water requirements

The quantity and quality of water made available to your livestock is also an important factor for production. Water intake will depend on a range of factors such as the breed, class and life stage of livestock, feed quality, the distance between feed and water and environmental factors (season, temperature, exposure to sun). Table 1 describes the daily volume of water required by different classes of cattle.

The quality of water depends on the dissolved salt content (or salinity level, which is sum of all mineral salts in the water including sodium chloride, calcium and magnesium sulphates and bicarbonates), sediment levels and freshness. High pH and mineral content can also affect water consumption and in some cases, reduce appetite or cause animals to perish.

Salt tolerances for cattle, described as maximum mg/L of soluble salts, are as follows:

• 10 000 mg/L - overall maximum 
• 4 000 mg/L - maximum to ensure healthy growth
• 5 000 mg/L - maximum to maintain current condition 

Note: 1 000 mg/L = 1,785 EC. 1 mg/L = 1ppm

Cattle require less water when green pasture is available but require more during the summer months or during drought when more roughage is consumed and additional water is required to break it down in the rumen. Access to shade, especially during the summer months, can reduce water demand greatly as it minimises the need for animals to use evaporative cooling to control their body temperatures. Livestock may also avoid warm water so troughs and pumps should be maintained in good working order during summer to help ensure fresh water is available.

Where water is saline, livestock tend to drink greater quantities to enable their bodies to regulate the salt balance in their bodies. Water quality is especially important when the pasture consists of species with a high salt content such as saltbushes and other forbs and shrubs common to the arid zone. The quantity of water consumed by cattle will be higher when they are grazing on these shrubs, as more water is required to flush the extra salt out of their system. Where saline water and high salt content pastures occur, livestock may not be unable to flush enough salt through their system and therefore feed intake may be reduced and production will be affected.

Some plants contain toxic levels of salt and when these are grazed, any weight gain by animals may be due to the increase in water consumption rather than growth. Metabolising the excess mineral salts also requires a great deal of energy and so production will further suffer.

Stock water consumption can increase by 80% in situations where stock are drinking saline water in summer.

Table 1: Average daily water requirements for different classes of cattle.

 

Forage for production

Livestock in arid environments source the majority of their nutrition from grasses (both annual and perennial species) and annual herbs and forbs. These are the principle drivers of animal production, including maintenance, growth, fertility and milk production. Rain or flood events that contribute to more seasonal patterns of pasture growth will lead to improved animal production. As these (highly digestible and palatable) feeds are grazed or dry off, perennial shrubs are often grazed more and therefore provide valuable nutrients including protein when grasses and forbs are less abundant. Perennial shrubs however, are mostly only sufficient to maintain condition of cattle, rather than contribute to significant growth. This is because perennial shrubs tend to have a lower level of digestibility than grasses and forbs and contain a greater amount of material that is not readily available to meet the nutritional requirements of cattle.

Plant selection by stock

Cattle may be selective about what they graze and do not necessarily eat anything or everything, especially if they have a choice. The selection of grazing plants will depend on the palatability of a particular plant. The palatability of a plant can provide a guide about how desirable a pasture species may be to cattle. Leafiness and the protein content of leaves are key determiners of a plant's palatability, and these vary between pasture species.

Variation in palatability can also occur between individual plants of the same species: you may have noticed that some shrubs will be more heavily grazed but the plant growing next to it has not even been nibbled. This may be due to slight differences in the palatability or digestibility between the two plants.

While not all the reasons for plant selection by cattle are known, it is known that the life stage of plants affects palatability. As a plant matures it becomes less digestible. Leaves are more digestible than stems and twigs, which are more digestible than branches. There are many palatable shrubs that will not be grazed if they are flowering or seeding, or if only old growth is present. Therefore regular grazing of perennial grasses and shrubs is likely to encourage production of new leaf and soft stems/twigs.

Plant selection by livestock (and native grazers) is a key contributor to the reduction of desirable species and the increase in density of undesirable species (especially shrubs), even in well-managed landscapes.

Remember that even if a species is highly nutritious in terms of its digestibility, it will have little feed value if it is not palatable or preferred by livestock.

Bite selection by cattle

Bite selection by cattle is influenced by the way they physically eat. When cattle eat vegetation, their tongue sweeps out in an arc, wraps around the plant parts, then pulls them between the teeth on the lower jaw and a pad on the upper jaw. The animal then swings its head so its teeth can sever the material.

Conversely sheep and goats do not extend their tongues, but rather “bare” their teeth and use them to cut the plant material. This means sheep and goats take smaller bites than cattle, can be more selective and can sever plants closer to the ground.

As a result, cattle tend to preferentially select soft leafy material rather than tough leaves, twigs, stems and branches, as sheep and goats do. However if a leafy grass sward is not available, cattle will resort to eating these tougher plant plants.

The rumen

Ruminant livestock such as cattle have a complex digestive system comprising four compartments: the rumen, the reticulum, the omasum and the abomasum (true stomach). Our attention will be focused on the rumen and the abomasum.

The rumen is extremely important to the nutrition of the animal, typically producing 60-80% of the ruminant’s energy in the form of volatile fatty acids (VFAs).

The rumen may be thought of as a fermentation vat, where rumen bugs (microbes) break down fibre (the plant cell walls etc.) into VFAs, which are then absorbed into the bloodstream across the rumen wall. A further advantage of the rumen is that as the microbes multiply they utilise non-protein nitrogen (eg urea), converting it to a form that can then be utilised by the animal when the microbes themselves are digested (microbial protein).

As the name suggests, the ‘true stomach’ is most similar in function to the stomach of humans and other monogastric animals, as it is where food matter is broken down by acidic digestion rather than fermentation.

Rumen fermentation processes create a large amount of gas which is expelled through the mouth by eructation (burping). Ridding the digestive system of these gases is vital to the health of livestock as severe cases of rumen bloating can be fatal.

Ruminants eat rapidly, swallowing much of their feedstuffs without chewing them sufficiently. The oesophagus functions bi-directionally in ruminants, allowing the animals to regurgitate food for further chewing if necessary; this is known as “chewing the cud”. In the process of chewing the cud, more saliva is mixed with the food improving its digestion. The cud is then re-swallowed and passed into the reticulum for further digestion.

  

Figure 1: Ruminants have a complex digestive system

Common nutrition terms

Following are some of the most common terms used when discussing livestock nutrition.

Dry matter (DM): Fodder – water = Dry Matter

Dry matter is simply what is left of a plant when the water is removed. As the water content of plants is highly variable, DM is used as a standard term for animal feed and is expressed as a percentage of the total amount consumed. Dry matter contains the energy, protein, vitamins and minerals required by livestock for maintenance and production.

Dry matter intake (DMI): Dry matter – faeces = Dry matter digestibility

Dry matter intake is the amount of dry matter ingested by an animal per day, expressed as a percentage of live weight. DMI is influenced by a many of factors including the diversity of feed available, selective grazing of preferred species and the presence and concentration of secondary compounds in plants. In an extensive shrub lands such as in the arid zone, DMI has greater importance as the level of sodium and secondary compounds can be high and this may limit the DMI of your cattle.

Dry matter digestibility (DMD): Dry matter – faeces = Dry matter digestibility

Dry matter digestibility is the percentage of DMI that can be digested. This does not include matter that is not digested by the animal and passes as faeces. For example, if an animal consumes 5 kilograms of dry matter and passes 1 kg of dry matter, then the DMD is 4 kg (or 80%). This is the material that is used by the animal for survival and production processes.

Dry organic matter digestibility (DOMD)

Dry organic matter digestibility is the proportion of the organic matter in the digestible dry matter that can be digested by an animal. This is calculated by incinerating the dry matter and measuring the remaining ash, which is indigestible.

Metabolisable energy (ME)

Animals gain energy from plant material they consume. Not all energy will be used for production by animals, as it is lost to production through waste products including faeces, urine, gases.

The remaining energy, or metabolisable energy (ME), is made available to the animal for production or is lost as heat. ME is calculated from the DMD and is expressed as megajoules per kilogram of dry matter (MJ/kg DM). You may also see the term net energy. Net energy is energy that is only used for production i.e. ME minus the energy lost as heat.

ME will differ between plant species but also between different parts of the one plant. Energy requirements of sheep and cattle vary depending on a number of factors including age, sex, seasons, stage of growth and level of activity.

Crude protein (CP)

Crude protein is a measure of the nitrogen content of feed and includes protein (true protein) and non-protein nitrogen (NPN). All protein is absorbed from the small intestine, however as previously described, there is an additional source of protein available to ruminants in the form of microbial protein synthesised in the rumen.

The protein consumed by the animal is (broadly) categorized into two groups:

• rumen degradable protein (RDP)
• rumen undegradable protein (RUP)

The RDP includes true protein and NPN and is available to rumen microbes. Eventually RDP passes into the abomasum as microbial protein. If there is not adequate RDP in the diet the ability of the rumen (and rumen microbes) to function will suffer. Therefore the energy available to the animal may be compromised and animal performance would be expected to suffer.

The UDP, as the name suggests, cannot be utilised by the rumen microbes and is therefore passed by the animal in the same form that it was consumed by the animal. It is sometimes referred to as bypass protein.

Crude protein is considered an inaccurate measure of protein in intensive grazing systems where feed can be custom-made. This is due to the fact that crude protein can include a measure of protein which is ‘bound’ and is not available to livestock (or the rumen microbes). For extensive grazing systems such as those in shrub lands, crude protein is usually an adequate measure, given that the feed on offer in these systems is quite diverse.  However if you have any concerns regarding the dietary protein available to your animals, you should discuss them with an animal nutritionist.

Neutral detergent fibre (NDF)

Neutral detergent fibre is the structural components of the plant, such as the cell walls. It provides the bulk of the feed volume consumed and is slowly digested. NDF affects daily dry matter intake by cattle: the higher the level of NDF in the diet, the lower the intake. This is essentially because the feed contains too much fibre, which slows down digestion and limits intake. It is important to note that as a plant matures NDF increases.

Table 2: Nutrient requirements of cattle at different life stages (Adapted from NRC, 2000 and 200)

 

 

 

 

 

 

 

 

 

 

 

Acid detergent fibre (ADF) 

This is the least digestible part of the plant. Plants with low ADF are often higher in energy.

Minerals

Macro minerals are the primary and essential minerals for sound animal health and production and are required in larger quantities than micro minerals. 

Micro minerals (or trace elements) are also essential for animal health but are required in relatively small quantities. They include chromium, cobalt, copper, iodine, iron, manganese, molybdenum, nickel, selenium and zinc.

Table 3: Mineral requirements of cattle