Nutrient Requirements Nonruminant Herbivores

Michael R. Murphy Amy C. Norman

University of Illinois at Urbana Champaign, Urbana, Illinois, U.S.A.

INTRODUCTION

Nonruminant herbivorous mammals include a small number of commercially important animals and a larger number of wild species.[1] Digestive strategies clearly differ among these herbivores. Mammals lack enzymes to hydrolyze a substantial portion of plant material (cell walls), but various pregastric (including ruminant) and postgastric microbial fermentation systems have evolved that enable herbivorous mammals to utilize fibrous substrates. Digestive strategy and body size data for East African nonforest herbivores indicated that ruminants dominated medium body sizes, whereas nonruminants prevailed among very large and small herbivores[2] (Fig. 1).

Our objective was to briefly review current knowledge about the nutritional requirements of nonruminant herbivores. Those for horses (Equus caballus) and domestic rabbits (Oryctolagus cuniculus) are stressed. Among commercially important and widely distributed species, horses and rabbits represent very large and small mammalian herbivores, respectively. In addition, they exemplify subgroups of postgastric fermenters that emphasize colonic (horses) or cecal (rabbits) function. More detailed information is also available on their nutritional requirements than for many other species.

HORSES Water

Horses usually drink 2 to 3 L of water/kg of dry matter consumed. Water intake increases with lactation, exercise, and elevated temperatures by 50 to 70%, 20 to 300%, and up to 300%, respectively. Ad libitum access to fresh, clean water is recommended except after intense exercise, when horses should be allowed to drink only small amounts every 5 to 10 minutes for approximately 1 hour.[3]

Energy

Horses get most of their dietary energy from carbohydrates and lipids. Energy value is usually expressed in terms of digestible energy (DE, gross energy minus fecal energy).[4] Structural carbohydrates, such as cellulose and hemicellulose, often make up the majority of their diet[3] and are fermented by microbes in the cecum and colon to provide much of the energy required by a horse at maintenance.[4] A minimum of 12 to 15% fiber is presumed necessary to minimize incidence of colic and laminitis, but forages alone do not generally provide sufficient energy for growing, working, or lactating horses, so cereal grains are added to their diets. Cereal grains provide digestible nonstructural carbohydrate (starch).[4] Lipids may also be supplemented, providing 2.25 times the energy value of carbohydrates,[5] and 20% added fat can be included in the diet without adverse effects.[4] Diets supplemented with fat should be monitored closely for rancidity, because spoiled feed is not accepted. Supplementation with fat improves work output, reproductive performance, milk production, and foal growth, but it must be monitored closely to avoid obesity and insulin resistance.1-41

Protein

Amino acids, the building blocks of protein, are required.[4] Protein deficiency retards growth of young horses and causes tissue loss, poor coat, and abnormal hoof development in the adult. Average protein intake at maintenance is approximately 0.6 g of digestible protein/ kg/day and should be increased during late gestation and early lactation. Protein requirements for working horses have not been clearly defined, but it is not considered advantageous to feed protein above the maintenance requirement. High-quality protein is essential for the growing horse, and it appears that growth is maximal when the protein-to-energy ratio is 50 and 45 g of crude protein/Mcal of DE/day for weanlings and yearlings, respectively. Lysine is the first-limiting amino acid for growing horses, and there appears to be no beneficial effect of including nonprotein nitrogen sources in practical diets for horses.[4]

Minerals and Vitamins

The major minerals needed by horses are Ca, P, Na, K, Cl, I, Fe, Cu, Zn, Mg, and Se.[4] Bone is approximately 35%

CA U

SP 40

0.01 0.1 1 10 100 1000 10000 Size, kg

—•— Nonruminants - O - Ruminants ♦ Rabbit ■ Horse

Fig. 1 The relationship between digestive strategy and body size in 186 species of East African nonforest herbivores. (Adapted from Ref. 2, with the sizes of rabbits and horses marked for comparison.) (View this art in color at www. dekker.com.)

toxicity. Iron is adequate in most diets, so supplementation is unnecessary, although frequently practiced.

Vitamins are often classified as fat-soluble or water-soluble. The former category includes vitamins A, D, E, and K. Vitamin A is important for good vision. Vitamin D is essential for calcium and phosphorus absorption, but rarely needs to be supplemented if animals are exposed to sunlight. Water-soluble thiamin and riboflavin are discussed in a publication of the National Research Council (NRC).[4] A deficiency of thiamin can cause a multitude of problems, but neither deficiency nor toxicity of ribo-flavin has been reported. Requirements for other water-soluble vitamins (niacin, pantothenic acid, pyridoxine, biotin, folacin, B12, ascorbic acid, and choline) have not been determined, but they are presumed to be required. Table 1 summarizes nutrient requirement data for horses.

0.01 0.1 1 10 100 1000 10000 Size, kg

Ca and 16% P. The dietary Ca:P ratio is critical for proper bone development; ratios less than 1:1 can impair Ca absorption and cause detrimental bone abnormalities in developing horses. Sodium, K, and Cl are the three major minerals involved in electrolyte balance, and it is necessary to maintain proper concentrations of each. Iodine is important for regulation of metabolism, but it should be closely monitored because horses are susceptible to iodine

RABBITS

Mature rabbits vary greatly in size, from 1 to 6 kg.[6] Therefore, their nutrient requirements are not usually specified on an amount-per-day basis, but on a dietary concentration relative to body size, or relative to metabolic body size basis.

Table 1 Estimated nutrient requirements for a 500 kg mature horse

Nutrient

Unita

Growth

Maintenance

Gestation

Lactation

Work

Water

L

23 30

38 45

38 57

38 57

38 68

Energy

Mcal of digestible energy

14 19

16.5

18 19

28 24

20 33

Protein

g

720 850

656

801 866

1,427 1,048

820 1,300

Minerals

Calcium

g

34 29

20

35 37

56 36

25 40

Phosphorus

g

19 16

18

26 28

36 22

18 29

Potassium

g

11.3 17.8

25

29.1 31.5

46 33

31.2 49.9

Magnesium

g

3.7 5.5

7.5

8.7 9.4

10.9 8.6

9.4 15.1

Sulfur

%

0.15

0.15

0.15

0.15

0.15

Sodium

%

0.1

0.1

0.1

0.1

0.3

Iron

mg/kg

50

40

50

50

40

Manganese

mg/kg

40

40

40

40

40

Zinc

mg/kg

40

40

40

40

40

Copper

mg/kg

10

10

10

10

10

Selenium

mg/kg

0.1

0.1

0.1

0.1

0.1

Iodine

mg/kg

0.1 0.6

0.1 0.6

0.1 0.6

0.1 0.6

0.1 0.6

Cobalt

mg/kg

0.1

0.1

0.1

0.1

0.1

Vitamins

A

IU/kg

2,000

2,000

3,000

3,000

2,000

D

IU/kg

800

300

600

600

300

E

IU/kg

80

50

80

80

80

aAmounts or concentrations on a dry matter basis. (From Refs. 3 and 4.)

aAmounts or concentrations on a dry matter basis. (From Refs. 3 and 4.)

Water

Although the NRC[6] did not address the subject of water, others[7'8] have noted that the water requirements of rabbits fed dry feed far exceed their dry matter intakes. Consumption of such diets drops precipitously if water is withheld. Water intake on dry diets is about 120 mL/kg of rabbit, or twice the amount of feed consumed. Environmental temperature also influences water consumption, increasing it by 67% between 18 and 30°C. High-quality drinking water should always be available.

Energy

For diets containing 12 to 15% digestible protein, DE and metabolizable energy (ME, DE minus urinary energy in nonruminants) are closely correlated, and ME is about 95% of DE. Diet ME and net energy (ME minus heat increment) contents are more difficult to determine than DE, so DE values are still commonly used in practical rabbit feeding.[8]

Rabbits do not utilize plant fiber as efficiently as widely assumed[6] and coprophagy (consumption of soft feces of cecal origin) does not appear to greatly influence the overall efficiency of fiber digestion.[7] Cellulose and hemicellulose digestibilities in rabbits are similar to those of rats, and less than in horses and guinea pigs. Only about 10% of neutral detergent fiber in timothy hay was digested by rabbits, compared to about 35% for horses and ponies. Rabbit growth rate is apparently optimal with diets having 13 to 25% acid detergent fiber. A minimum of 10% dietary crude fiber is needed to maximize growth rate (and to prevent enteritis and fur pulling), but over 17% depresses growth by restricting feed intake.

Starches, sugars, and lipids apparently pose no special problems for rabbits. The likelihood of a deficiency of essential fatty acids is remote, but it has been demonstrated in rabbits.

Protein

Rabbits need adequate quantities of essential amino acids in their diet for rapid growth, and nonprotein nitrogen cannot be employed usefully in grower diets.[6] Protein quality must allow essential amino acid requirements to be met. Required and optimal concentrations of some amino acids have been established for growing and lactating rabbits.[6-8] Rabbits are able to utilize 64 to 90% of the crude protein in common feedstuffs.[7] They can maintain positive nitrogen balance when fed gelatin, a protein devoid of the essential amino acid tryptophan, because of the consumption of microbial protein via coprophagy. Negative nitrogen balance occurred when coprophagy was prevented. Increased feed intakes can compensate for low protein concentrations in diets. Therefore, it is desirable to express protein requirements per unit of energy. Growth is optimized with about 55 mg of crude protein/kcal of DE.

Minerals and Vitamins

The rabbit is unusual because serum Ca concentration reflects dietary Ca concentration, rather than being homeostatically regulated in a narrow range as in other species.[6,7] Hypocalcemia is sometimes observed in late gestation or early lactation. It is treatable with Ca-gluconate injection. However, whether an acidotic diet during late gestation would be prophylactic, as it is for a dairy cow, is not known.[8] Requirements for many minerals have not been well studied, although deficiencies and problems with excesses have often been demonstrated.

Vitamin A deficiency and toxicity have been demonstrated, but precise requirements have not been deter-mined.[7,8] Any dietary requirement for vitamin D is likely

Table 2 Estimated nutrient requirements for rabbits (amounts are per kilogram of air dry diet, unless otherwise specified)

Nutrient

Unit

Growth

Lactation

Water

kg

1.6

2.0

Energy

kcal

2,500

2,500

MJ

10.5

10.5

kJ of digestible

950

1,200

energy/kg075

Protein

g

170 180

170 180

Minerals

Calcium

g

8

11.8

Phosphorus

g

5

6.6

Potassium

g

6

9

Sodium

g

2

2.2

Chlorine

g

3

3.2

Magnesium

g

3

3

Iron

mg

50

75

Zinc

mg

25

50

Copper

mg

10

10

Manganese

mg

8.5

10

Iodine

mg

0.2

0.2

Cobalt

mg

0.1

0.1

Selenium

mg

0.01

0.01

Vitamins

A

IU

6,000

10,000

D

IU

1,000

1,000

E

mg

35

45

K

mg

1

2

(Mean or median values compiled from Refs. 6 8.)

(Mean or median values compiled from Refs. 6 8.)

much lower than for other species. The only practical problem encountered with vitamin D in rabbit nutrition is toxicity: 2300 to 3000 IU of vitamin D/kg are detrimental. Vitamin E deficiency has been demonstrated, but recommendations are based primarily on old data or extrapolation from other species. Vitamin K is probably not of practical concern in rabbit nutrition because it is synthesized in the cecum, and no requirement studies have been conducted.

Under practical conditions, B-complex vitamins are not dietarily essential for rabbits, but deficiencies have been demonstrated. Addition of B vitamins to commercial rabbit feeds has not shown benefits. Rabbits can synthesize vitamin C, so it is not a dietary essential either. In commercial diets, it is advisable to include a vitamin mixture that provides at least moderate concentrations of vitamins A and E to ensure that no deficiency occurs. Table 2 summarizes nutrient requirement data for rabbits.

CONCLUSION

Much remains unknown about the nutritional requirements of nonruminant herbivores. Current data, however, allow many practical dietary limitations and toxicities to be avoided in commercially important and widely distributed species, particularly horses and domestic rabbits.

REFERENCES

1. Cork, S.J.; Hume, I.D.; Faichney, G.C. Digestive Strategies of Nonruminant Herbivores: The Role of the Hindgut. In Nutritional Ecology of Herbivores; Jung, H. J.G., Fahey, G.C., Jr., Eds.; Amer. Soc. Anim. Sci.; Savoy: IL, 1999; 210 260.

2. Demment, M.W.; Van Soest, P.J. A nutritional explanation for body size patterns of ruminant and nonruminant herbivores. Am. Nat. 1985, 125, 641 672.

3. Lawrence, L. Feeding Horses. In Livestock Feeds and Feeding, 5th Ed.; Kellems, R.O., Church, D.C., Eds.; Prentice Hall: Upper Saddle River, NJ, 2002; 381 401.

4. National Research Council. Nutrient Requirements of Horses, 5th Rev. Ed.; Natl. Acad. Sci.: Washington, DC,

1989.

5. Ensminger, M.E.; Oldfield, J.E.; Heinemann, W.W. Feeds and Nutrition, 2nd Ed.; Ensminger Publ. Co.: Clovis, CA,

1990.

6. National Research Council. Nutrient Requirements of Rab bits, 2nd Rev. Ed.; Natl. Acad. Sci.: Washington, DC, 1977.

7. Cheeke, P.R. Rabbit Feeding and Nutrition; Academic Press: Orlando, FL, 1987.

8. de Blas, C.; Wiseman, J. The Nutrition of the Rabbit; CABI Publ.: New York, 1998.

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