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Dietary lipid and cholesterol are reported to play a major role in reproductive performance of ostriches and are thus influential on flock expansion. The omission of fat in feed is detrimental to health due to an inability to utilise fat-soluble vitamins. A blend of animal fat and vegetable oils has been shown to improve fat mobilisation.

Fat colour is important in gauging the general health and meat quality from the birds. The avoidance of yellow fat in red meat production is seen as important due to its lack of consumer acceptance as a result of its appearance and, sometimes, unpleasant odour. Yellow fat has erroneously been considered not important in the ostrich, as it has no fat trim or marbling. It is, however, an indicator of dietary imbalances. This emphasises the need for a balanced ration in order to maximise body energy and protein gain, and of the beneficial influence of supplemental fats on energy utilisation from non-dietary lipid constituents. The development of such rations can be achieved most effectively through the joint efforts of producers and scientific researchers.

APPROPRIATE NUTRITION VITAL
Adequate nutrition is vital for good ostrich productivity[13]. Ideally knowledge of the specific nutrient requirements of ostriches is needed to achieve maximum growth rates[28] and good meat yields [6]. Indeed, good quality meat is vital if the ostrich farmer is to maximise his return on investment[11],[12]. Within the domestic birds, the current literature of lipid utilisation is restricted mainly to poultry, much of which discusses the effect of dietary inclusions on lipid balance[1],[2],[4],[9],[20] and the provision of metabolisable energy[23].

Dietary lipid and cholesterol have been reported to play a key role in the reproductive performance of poultry[14],[15],[19],[32] and are thus likely to be important in ostrich productivity. The correct balance of protein and carbohydrate in the diet is important given their association with body fat content[29] via hepatic metabolism. Imbalances are liable to lead to the deposition of yellow fat [6], and can arise from sudden changes of dietary fat sources[25]. The aim of this review, therefore, is to discuss the importance of lipid in the diet of the ostrich, and its role in productivity via its influence on growth and development.

FAT DEPOSITION IN OSTRICHES
Fat deposition in ostriches is as a result of hepatic mobilisation of fatty acids and glycerol. There are currently only a limited number of reports on dietary fat digestibilities in ostriches. In one study the feed used contained 7.3 % fat (Soya bean oil) and 33.9 % neutral detergent fibre (NDF), the digestibilities of which were shown to increase with age[3] (Table 1).

Table 1: Apparent NDF and fat digestibilities in ostriches of different ages

Means with different superscripts are significantly different (p<0.05)
Source: Angel (1993)

Studies of nutrient mobilisation in Ratites are limited. One study[31] makes an attempt to discuss nutrient utilisation in emus, although the data presented therein cannot be related directly to ostriches, because of anatomical differences between these two Ratite species[27]. It has been reported that the omission of animal fat in feed formulations is detrimental to the utilisation of fat-soluble vitamins[18]. The author describes the mobilisation and storage of vitamins A, D3, E and K as being most efficient in the natural body fat of the ostrich, being efficiently achieved with a dietary blend of vegetable fat and animal fat.

Unfortunately, many manufactures of ostrich feed do not observe fat levels adequately. Observing a balance of quality animal and vegetable fat in the ration at levels to match the nutrient levels in the ration, ensures the efficient carriage of fat-soluble vitamins into the bloodstream on a daily basis. In the laying ostrich hen, stress may arise from high production or extreme weather, resulting in the mobilisation of its fat reserves and fat-soluble vitamins for the development of eggs and embryos.

It is reported in poultry that most lipid in egg yolk is formed in the liver by using fatty acids obtained from the diet or from novo synthesis. Therefore the provision of dietary fat decreases the need for hepatic fatty acid synthesis and generally increases yolk formation and weight of the egg. Indeed, it has been reported by the NRC Nutrient Requirements for Poultry (U.S.A.) that vitamin A is important in egg production, hatchability and fertility; vitamin D in egg production, hatchability, fertility and shell quality; vitamin E in hatchability; and vitamin K in hatchability.

The addition of fat to the layer diet in turkeys has been reported to result in an increased egg production, fertility and body weight[15]. Other studies in poultry describe a significantly (p<0.01) improved fat mobilisation in birds when vegetable oils, and animal-vegetable blend fat is added to their diets[21]. In this study, diets containing no supplemental fat acted as controls, against which diets containing 5 % of tallow, corn oil, soybean oil, animal-vegetable blend fat, or canola oil were compared. However, in birds fed diets containing palmitic acid, oleic acid or a 50:50 (wt/wt) mixture of these fatty acids, there was a significant (p<0.05) reduction in the apparent retention of nitrogen, magnesium and calcium. This was confirmed in an earlier study[5] in which metabolisable energy values of diets followed a trend similar to fat retention and independent of dietary calcium levels. Hence the importance of ensuring a combined animal-vegetable fat mixture to the diet.

FAT COLOURING
Many studies fail to note the fat colour, liver condition and meat yield in their birds. It is often considered that the yellow colour is simply caused by the use of maize and/or Lucerne, or grass in the rations and it is the beta-carotene in these ingredients that is responsible for the yellow colouration[26], making many believe that fat colour is not a significant problem. The significance of fat colour is because it is one indicator as to the general health of the birds and can be used as a dependable indicator of meat quality and consistency from the birds[24].

There are also many references in other animals of lower meat yields due to poor muscle development. For instance, grass-fed cattle consistently achieve a lower grade to those fed a balanced ration for a number of days prior to slaughter, and yellow fat is perceived as a significant problem[22]. Lower meat yields go hand in hand with carcasses that have yellow fat. Experiments with grass-fed cattle being supplemented for a period prior to slaughter have experienced improved muscle growth, fat and meat colour, and meat yield[24],[26], implying that ration imbalances and/or nutrient deficiencies are prevalent in cattle grazing. Indeed, grazed animals rarely have access to adequate vitamin and mineral supplementation resulting in deficiencies of essential nutrients, and, like grazing cattle, ostriches being fed a deficient diet, will experience lower meat yields and develop yellow fat.

From a farmer's point of view, the presence of white fat is a useful "clue" as to the adequacy of his/her rations and his/her ability to maximise feed conversion in his/her birds[22]. What is not understood by many is that animals draw on their fat reserves during times of stress[16]. Whereas white fat is very easily mobilised and can be quickly converted into energy, yellow fat is tightly bound and mobilises much more slowly especially when nutrient deficiencies are severe[6]. When fat is easily mobilised, a healthier bird will result especially during the winter months when slaughter birds are growing the fastest, and require additional energy during bad weather.

CONTROLLING FAT PRODUCTION
There are many components in the diet that control fat production and mobilisation. It is well documented that an excess of energy in a diet will cause excess fat to be laid down and that an excess of protein will convert to fat. One study demonstrated that in chicks fed diets deliberately void of protein ingredients, carbohydrate and fat utilisation were significantly (p<0.01) diminished[30]. The response surface of body energy gain was roughly parallel to that of metabolised energy, the former being dependent on the caloric ratio of dietary carbohydrate to fat. Another study reports that the addition of a relatively unsaturated fat or saturated fat to the diet enhances the dietary metabolisable energy[23]. The authors also describe the inclusion of fat in the diet as increasing the utilisation of energy from sucrose.

These studies emphasise the need for a balanced ration in order to maximise body energy and protein gain, and of the beneficial influence of supplemental fats on energy utilisation from certain non-dietary lipid constituents. An excess of these nutrients may be caused through the inclusion levels in the ration being too high, but more often, the cause of excesses is a shortage and/or imbalance of vitamins and/or minerals in a ration that results in the animal's inability to utilise the nutrients in the rations.[8],[10] It has been reported that low phosphorus levels contribute to a poor utilisation of the high energy and causes even greater fat production[17].

Calcium, phosphorus, zinc, manganese, copper, selenium, magnesium, potassium, and salt are also important minerals/trace minerals that assist with the total digestion process and are key to fat and meat production. Phosphorus, for instance, increases energy utilisation by helping livestock make better use of carbohydrates in the rations. Vitamins A, D3, E and K also help with the digestion/conversion process via their general effect in promoting weight gain and feed efficiency. In addition, the B-complex vitamins including Choline, Niacin and Biotin, help convert body fat to mobilised energy in the bird.

The B-complex vitamins control or regulate enzyme activity in the body, and are principally involved in the breakdown of feed nutrients for absorption into the bloodstream. As such, these enzymes stimulate appetite, and promote an increase in production, more efficient feed utilisation and improved reproduction. Deficiency symptoms are often severe; for instance, chicks deficient in Niacin show poor feathering, scaly dermatitis and sometimes a "spectacled eye". Certainly, if the bird has some body fat but cannot mobilise it, it just gets fatter[17] with undesirable effects on meat quality[11].

CONCLUSIONS
Minerals, trace minerals and vitamins must be balanced with the rest of the ingredients in the ostrich diet in order to maximise metabolic utilisation. Indeed, diets can be carefully formulated so that they control the amount of fat desired and enhance meat yields[7]. The importance of using common sense in the development of a productive ratite diet by the producer through observation, understanding and experimentation, should not be overlooked as this is often crucial to successful flock growth. Working hand-in-hand with scientific researchers in order to develop a more practical focus for improving ostrich farming is also important.

REFERENCES
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2. An BK, Nishiyama H, Tanaka K, Ohtani S, Iwata T, Tsutsumi K, Kasai M 1997 Dietary safflower phospholipid reduces liver lipids in laying hens. Poultry Science 76(5): 689-695

3. Angel CR 1993 Research update. Age changes in the digestibility of nutrients in ostriches and nutrient profiles of the hen and chick. Proceedings of the Association of Avian Veterinarians: 275-281

4. Asghar A, Lin CF, Gray JI, Buckley DJ, Booren AM, Crackel RL, Flegal CJ 1989 Influence of oxidised dietary oil and antioxidant supplementation on membrane-bound lipid stability in broiler meat. British Poultry Science 30(4): 815-823

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7. Benson FV 1999b Fat colour, meat colour indicates the quality of the product. Ostrich Farmer Forum 3 (17): 2,6

8. Benson FV 1999c Feeding for production, quality and profit. European Ostrich Association World Congress, Albufeira, Algarve, Portugal, 5-7 November 1999: 8

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10. Cilliers FC 1999 Basic concepts and recent advances in scientifically feeding of ostriches. European Ostrich Association World Congress, Albufeira, Algarve, Portugal, 5-7 November 1999: 21

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12. Cooper RG 1999b Critical success factors for the Zimbabwean ostrich industry. MBA dissertation, Nottingham Trent Business School, Nottingham Trent University, Nottingham

13. Cooper RG, Benson FV 1999 Soyabean meal, an important component of ostrich diets. World Poultry - in press

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27. Scheider SE, Angel R 1994 Feeding big birds. Feed International May: 22-26

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29. Toyomizu M, Akiba Y, Horiguchi M, Matsumoto T 1982 Multiple regression and response surface analyses of the effects of dietary protein, fat and carbohydrate on the body protein and fat gains in growing chicks. Journal of Nutrition 112(5): 886-896

30. Toyomizu M, Akiba Y, Matsumoto T, Hotiguchi M 1985 Response surfaces of body protein and energy gains in growing chicks fed diets over the entire range of compositions of protein, fat and carbohydrate. Journal of Nutrition 115(1): 61-69

31. Waterhouse HN 1996 Emu guidelines can be made, more research needed. Feedstuffs 69(18): 1-4

32. Whitehead CC, Bowman AS, Griffin HD 1991 The effects of dietary fat and bird age on the weights of eggs and egg components in the laying hen. British Poultry Science 32(3): 565-574

Authors:
a Department of Physiology, University of Zimbabwe, P.O. Box MP 167, Mount Pleasant, Harare, Zimbabwe. Fax: (263) (4) 333678. E-mail: rgcooper@hotmail.com
b Blue Mountain Feeds International, P.O. Box 267, Hopefield, 7355, South Africa. Fax: (27) (022) 723 2105. E-mail: fiona@blue-mountain.net
c Blue Mountain Feeds Inc., 2001 Blue Mountain Avenue, Berthoud, Colorado CO 80513, U.S.A. Fax: (1) 303 772 7853. E-mail: daryl@blue-mountain.net

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