Introduction

For dairy cattle to have high yield in producing milk, it is vital to have sufficient energy. Energy intake is very important for dairy cattle for several reasons. For example, sufficient energy is required for dairy cattle to increase milk production yield during lactation phase. Energy is also important to maintain the body weight of dairy cattle during lactating period.

There are many ways to supply sufficient energy to dairy cattle. Fat is very appealing due to its much higher energy density. It has two times the energy per unit weight compared to carbohydrate.

Biohydration and Digestion of Fat

Fat digestions begins in the rumen (Figure 1). The bacterial lipases then breaks down fat into glycerol and fatty acids (FA). Glycerol is used as an energy source by bacteria and is principally converted into propionic acid. The unsaturated FA are biohydrogenated by bacteria but the bacteria do not use them as an energy source. Biohydrogenation is not complete. Approximately 60 - 90% of the FA becomes hydrogenated. This will very much depend on the fat sources, the rate of particulate passage and ruminal conditions.

Adding fats to diets will help improve the efficiency of microbial protein synthesis in the rumen. Small intestine is where absorption takes place. Most of the FA flowing to the small intestine will be free fatty acid (FFA) but some glycerides and phospholipids will be present. Presence of pacreatic lipase will result in cleavage of the glycerides and phospholipids so the FA can be incorporated into the micelles for absorption. Bile is critical for emulsification of FA so that absorption can take place. The FA are reassembled as phospholipids and glycerides which is then packaged as very low density lipoproteins (VLDL) and chylomicrons which is then transported throughout the lympathic system.

Desirable characteristics of a fat source

Desirable fat source should have minimum effects on ruminal fermentation and high digestibility. There are basically two types of fats: 1) unsaturated fat 2) saturated fat. Unsaturated fat is highly digestible but may reduce fibre digestibility in the rumen due to inhibitory effects on cellulolytic microorganisms. Saturated fat on the other hand have less influence on fibre digestion in the rumen but is inferior in terms of digestibility depending on the level of saturation.

Other factors affecting digestion are chemical and physical form of the fats. For hydrogenated fats, the high IV (up to 27) and high ratio of C16:C18 will help improve the digestibility. The physical form of fats such as size particles will also affect digestibility. Saturated fats with smaller particle size (prills) as compared to the larger particle sizes (flakes) have better digestion. This is due to the larger surface area for absorption. As for the chemical form, it appears that fatty acid has higher digestibility compared to triglycerides.

The potential harmful effects of unsaturated fats from oilseeds may be minimised if the oilseeds are fed either whole or coarsely cracked rather than extruded. The free oil from oil seeds are not recommended to be used due to the rapid contact of oil with particulate matter and microbes. Grinding of soybeans is also not encouraged because of the reduced rumen undegraded protein in roasted soybeans and potential increase in oxidative rancidity with ground raw soybeans.

Fat Sources

Fat sources can be divided into 2 groups. The first is "natural fats" which can be further divided into plant fats and animal fats. The second group would be commercial or speciality fats. These are fat products made especially tailored for dairy cows.

Natural Fats

Oil seeds are a major source of plant fats. Whole cottonseed is considered a very well balanced feedstuff for dairy cattle as it is high in protein, fibre and has a high energy value. It is most easily handled by inclusion in a total-mixed ration. Cottonseed should also be closely monitored as there is possibility of mycotoxin contamination, especially aflatoxin.

Raw soybeans should not be grounded and added to feedstuff containing urea because soybeans contain urease. Soybeans can be included in total-mixed ration, top dressed, or in the case of cracked beans, be added to grain mixtures. By roasting soybeans, it will denature the urease, trypsin inhibitor and lipoxygenase and this will decrease the ruminal degradability of the protein. Overheating the beans will also reduce protein digestibility. Thus, it is important to avoid charred seeds. The colour of the seeds should be light brown.

Other oilseeds such as canola, safflower seeds and sunflower seeds can be fed to dairy cattle. However, it is very important to exercise caution when feeding other oilseeds mentioned as they are highly unsaturated, higher in oil compared to cottonseed and soybeans, thus resulting in lower recommended feeding levels.

Animal Fats

The main animal fat fed to dairy cattle is tallow. Tallow contains more saturated fatty acids compared to oilseeds. However, it is harder to handle them because it is in solid state at room temperature. The different grades of tallow are: edible tallow, extra fancy tallow, fancy tallow etc...

'Yellow grease' is a waste grease from the food and beverage industry. It can be used as fats for livestock or pets.

Other feedstuff such as hominy, dry distillers grain with solubles and fish meals contain a moderate amount of fats.

The most important factor to consider when feeding fats to dairy cattle is the total fat from all sources in the diet instead of only supplemental fat from major contributions.

Commercial Fats

Most of the commercial fats in the market are marketed as "rumen inert" fats or "bypass fats". There are basically 3 main ways to produce such products. They fall under:

1) Increasing the saturation of fatty acids so that the melting point is above the environmental temperature

2) Combining fatty acids with to form calcium soap

3) Fats "encapsulated" in several ways to physically prevent interaction with the ruminal microbes. More information can be found by reading fat supplements for dairy cows,

Quality Factors

Rendered or processed fats originated primarily as recovered waste fats. As such, they vary widely in quality. The few important factors when measuring the qualities of fats are:

• Solidification point (titre) • Saturation/unsaturation (usually measured by IV) • Total fatty acid (TFA) • Free Fatty Acid (FFA) • Moisture, insolubles and unsaponifiables (MIU)

Titre and IV values are both used to estimate unsaturation of fats. As for FFA, they are usually used to measure the amount of 'abused' to which the fat has been subjected. Good quality fats usually have low MIU, typically less than 1 percent.

Feeding Practices

In general, having a high energy intake during early lactation stage will result in higher milk yield. However, it is not advised to feed high levels of fat until 30 days postpartum. This is to allow the dairy cattle to adjust the lactational phase before fat is included in the diet. The main goal for the first 2 - 4 weeks of lactation stage is to maximise the dry matter intake rather than energy intake.

The need for fat in the dairy cattle diet should be based on the animal's milk yield, body condition, quality of forages in diet, and the level of DM intake by animals. As a general guideline, dairy cattle can efficiently utilise as much dietary fat as produced in milk, with appropriate adjustments for BW change. The amount of fat would very much depend on:

1) The fat source

2) Level of dry matter (DM) intake

3) Fibre level in the diet

Dairy cattle consuming higher amounts of DM and diets high in fibre can handle a higher level of dietary fats. Hence, it is encouraged that fat levels should be expressed as a percentage of DM intake rather than on weight basis since DM intake influences the amount of fat to include in diets.

In general, oilseeds can be added to the dairy cattle diet to provide approximately an additional 2% fat or tallow. Animal-vegetable blends on the other hand can provide roughly 2.5 - 3% of supplemental fat to diets. The benefits of commercial fat products become apparent when the total dietary fat level must exceed 5% of dietary DM. Commercial fats are also important for their rumen inertness and they are convenient due to their ease of handling. Other factors to consider when choosing fats include price, availability, characteristics that relate to palatability, inertness and digestibility.

Effects of supplemental fats on milk composition

Adding fats to diets of lactating cows will increase the milk yield and milk protein yield. It will also be able to increase the contribution of performed fatty acid, thereby increasing the long-chain fatty acid and decreasing the short-chain fatty acid in milk.

Conclusion

Fats are very important for increasing the energy density of diets for high producing dairy cattle. The physical and chemical properties of fat sources, dairy cattle milk yield, body condition and level of DM intake and associated costs are important factors to consider when feeding fats to dairy cows. The use of feed grade commercial fat products is only expected to increase because of the importance in increasing milk yield for dairy cattle.