Cow productivity index
Nutritive value milk
Economic value
Cow size
Conclusion
Bibliography
O. Syrstad
The author's address is: Norwegian Centre for International Agricultural Development (NORAGRIC), Agricultural University of Norway, PO Box 5002, N-1432 As, Norway.
Most of the livestock populations in the world serve more than one purpose. Cattle are kept worldwide for the production of milk and meat and, in many countries, also for traction power. Throughout Southeast Asia, buffaloes are used for the same purposes as cattle. In some production systems, cows rear their calves while at the same time their milk is being extracted for human consumption. In tropical countries, cattle must often be suckled by the calf in order to initiate milk let-down. Little, Anderson and Durkin (1991) found that partial suckling resulted in larger and healthier calves, longer lactations and greater offtake of milk (excluding milk fed to calves) compared with bucket feeding.
When small ruminants such as sheep and goats are kept for both milk and meat production, the young are usually allowed to suck all the milk from their dams for two to three months. After weaning, the dams are milked until milk secretion ceases under-the influence of the new gestation. In sheep, and sometimes also in goats, wool might be a more important product than either milk or meat. Manure, hides and blood are valuable byproducts from all species. In many societies, livestock also play an important role in religious and social life.
The scope of this article is restricted to the combined production of milk and meat. The terms used apply to cattle, but the arguments are equally valid for other species. The objective is to discuss procedures for comparing different genetic groups on the basis of their milk and meat outputs.
FAO, the International Livestock Centre for Africa (ILCA) and the United Nations Environmental Programme (UNEP) (1980) proposed a productivity index for comparing various breeds with respect to combined milk and meat production. The index considered reproductive rate (calving percentage), viability of cows and calves, weight of one-year-old calves, milk yield and body weight of cows. Reproductive rate, viability and weight of calves were used to calculate calf weight produced per cow per year.
Productivity indexes for cattle: an example
Exemples d'indice de productivité pour les bovins
Ejemplos de índice de productividad del vacuno
|
Parameter |
Shorthorn |
Zebu |
|
Cow viability (%) |
96 |
95 |
|
Calving percentage |
68 |
63 |
|
Calf viability to 1 year (%) |
80 |
65 |
|
Calf weight at 1 year (kg) |
90 |
120 |
|
Annual milk yield (kg) |
- |
71 |
|
Cow weight (kg) |
190 |
320 |
|
FAO/ILCA/UNEP (1980) procedure |
|
|
|
Productivity per cow per year |
50.0 |
58.1 |
|
Productivity index per 100 kg per cow |
26.3 |
18.2 |
|
Proposed procedure |
|
|
|
Productivity per cow per year |
50.0 |
85.9 |
|
Metabolic weight of cow (=W075) |
51.2 |
75 7 |
|
Productivity index per metabolic weight unit |
0.98 |
1.13 |
Source: FAO/ILCA/UNEP 1980.
* Calf weight per cow per year + annual milk yield.
Milk yield extracted per cow per year was transformed to its live-weight equivalent to a ratio of 9 (assuming that it takes 9 kg of milk to produce 1 kg of live weight in the calf) and added to the calculated calf weight. Finally, the result was expressed per 100 kg live weight of cow to take into account maintenance requirements.
Trail and Gregory (1981) used a similar index for comparing the merits of different cattle breeds and breed crosses in Kenya. They stated that "... this index is the most meaningful way to compare the actual productivity of the breed types, given the level of information available. Its merit lies in the fact that it relates all the more important production characters back to the actual weight of the breeding cow that has to be supported, which is closely associated with cow maintenance costs".
The ratio of 9 used to transform milk yield into its live-weight equivalent originates from Drewry, Brown and Honea (1959), who found that nine units of additional milk consumed by the calf resulted in one additional unit of live-weight gain. The use of this ratio for the transformation of milk to calf weight in a productivity index might be questioned. The procedure evaluates milk as animal feed, although actually it is a highly valued human food. It seems more appropriate to evaluate both calf weight and milk on the basis of their food value. This could be either their nutritive value or their economic value.
The nutritive value of a food commodity depends on its chemical composition. The composition of milk varies widely, according to breed, season of the year, stage of lactation, etc. When the milk output for a whole year is considered, most temperate breeds fall within the ranges of 3.5 to 5 percent fat, 3 to 4 percent protein and 4.7 to 5 percent lactose. High milk-fat content is usually associated with high protein content, but both are inversely related to milk yield. Tropical cattle have a higher milk-fat content than temperate breeds. Milk with 4 percent fat is widely used as a reference and this corresponds to about 3.4 percent protein and 4.8 percent lactose. The energy content of such milk is approximately 730 kcal per kilogram, i.e. 3.1 Mj. Milk also provides valuable vitamins, particularly A, B and D, as well as minerals, calcium and phosphorus.
The composition of milk obtained from milking partially suckling cows may differ from the composition of the whole milk produced by the cows. This occurs because the fat content of milk extracted increases in the course of milking, sometimes from less than 1 percent in the first portion to more than 10 percent in the last. If the calf is used to stimulate milk let-down and allowed to suck for some time, the milk left for the milker has an elevated fat content. Instead, if the calf is left to suck residual milk after milking, this milk will be very rich in fat and the fat content of the milk extracted at the next milking will be depressed. Even when the calf stays with its dam part of the time, e.g. during the day, a slight reduction in the fat content of the milk obtained at milking is to be expected since the calf empties the udder more completely than the milker is able to do.
Meat
Cattle carcasses vary in chemical composition much more than milk does, depending on breed, sex, age, level of nutrition, etc. Fat content may vary from almost zero to 30 percent and more. When a carcass is excessively fat, a large amount of the fat is trimmed off by the butcher and often not used for food. Paradoxically the fattest carcasses are found in countries where people are most keen to reduce the fat content of their diets. In most cases, however, the fat actually eaten would not amount to more than 10 to 15 percent of total carcass weight, i.e. 5 to 7.5 percent of live weight, assuming a dressing percentage of 50.
The protein content of a carcass is more stable than the fat content. On the basis of a comprehensive review of literature, Homb and Joshi (1973) reported that the protein content of the edible parts of-a carcass is in the range of 12 to 15 percent of chilled carcass weight. Protein content is negatively correlated to fat content. On average, five units of the increase in fat percentage is accompanied by a reduction of one unit in protein percentage. This is partly offset by an increase in dressing percentage, however. Watson (1943) found that the edible protein in a carcass is 8.5 percent of empty body weight, i.e. 7.5 to 8 percent of live weight of an animal with normal gut fill. Edible protein in blood and other offal can amount to 2 to 3 percent of carcass weight, or 1 to 1.5 percent of live weight. This brings the total amount of protein in nine edible parts to 8 to 9 percent of live weight. The carbohydrate content in carcasses is negligible. The energy content in food consumed amounts to 1 600 to 2 100 kcal per kilogram carcass (800 to 1 050 kcal per kilogram live weight). Meat is also a valuable source of some minerals (iron) and vitamins (B).
From this it can be concluded that 1 kg of live weight of calf provides 80 to 90 g of protein (8 to 9 percent) for human consumption. This is equivalent to the protein content in about 2.5 kg of milk (with 3.4 percent protein). In terms of energy, 1 kg of calf weight provides only 1.1 to 1.4 times that of 1 kg of milk (800 to 1 050 kcal and 730 kcal, respectively).
In a market economy, consumer preference for various commodities is reflected in the price. Food preferences are determined by palatability and social prestige, and the price ratio is highly influenced by the income level of the consumers. In some high-income countries, the price ratio of beef (whole carcass) to milk is as high as 10 to 1. In developing countries, this ratio is more in line with the nutritive value of the two commodities. Information from several African countries indicates that the price of 1 kg of beef (average for all parts of a carcass) is in the range of three to five times that of 1 kg of milk. This means that 1 kg of live weight of calf is equivalent to 1.5 to 2.5 kg of milk.
It may be argued that the value of the calf should not be assessed based on its beef value at one year, but rather on its potential value for beef production up to the optimum slaughter age. Sufficient data on growth and viability after one year are often not available, however, and predicting subsequent performance on the basis of yearling weight is unreliable. The growth rate from one year onwards is not closely associated with yearling weight, partly because the maternal effect, which is important at the early stages of life, disappears gradually with advancing age. Most of the advantage of faster growth after weaning is offset by a larger maintenance requirement. The effect of growth rate on feed efficiency is therefore only slight. All in all; the weight of calves at one year of age is considered a satisfactory measure of the value of the calf crop.
Another questionable point in the index proposed by FAO/ILCA/UNEP (1980) and that used by Trail and Gregory (1981) is the way cow maintenance is measured, i.e. by expressing output per 100 kg of cow weight. The maintenance requirement of an animal does not increase proportionally with body weight, but rather with body weight taken to the power of about 0.75 (often referred to as metabolic body weight). This means that an increase of 50 percent in body weight, e.g. from 300 to 450 kg, is accompanied by an increase of only about 35 percent in feed required for maintenance. Expressing output per 100 kg of body weight favours small cows and puts larger ones at a disadvantage. It seems fairer to calculate the output per unit of metabolic weight and use this as the index of productivity.
It appears from the above that 1 kg of live weight of a weaned calf is equivalent to about 2.5 kg of milk as a source of protein for human nutrition, while it corresponds to less than 1.5 kg of milk in terms of energy. In most of the countries where cows rear their calves at the same time as milk is being extracted for human consumption, both food energy and protein are in short supply. One kilogram of live weight of calf might be considered equivalent to about 2 kg of milk. The output might therefore be assessed as the live weight of calf after weaning (or at one year of age) plus one-half the weight of milk harvested. The productivity index should be the annual output per unit of metabolic weight of cow.
As an illustration, production data for shorthorn (West African) and zebu cattle in the Central African Republic were extracted from FAO/ILCA/UNEP (1980) and are presented here in the Table, together with the productivity indexes. It can be seen that the FAO/ILCA/UNEP index and the index proposed in this article rank the two breeds oppositely. While the former index judges shorthorn to be much more productive than zebu under the given conditions, the latter indicates a clear advantage for zebu, which is partly the result of more importance being given to milk yield and partly because the larger body size of zebu is penalized less in this index.
Drewry, K.J., Brown, C.J. & Honea, R.S. 1959. Relationships among factors associated with mothering ability in beef cattle. J. Anim. Sci., 18: 938946.
FAO/ILCA/UNEP. 1980. Trypanotolerant livestock in West and Central Africa, vol. 1. FAO Animal Production and Health Paper 20/1. Rome, FAO. 146 pp.
Homb, T. & Joshi, D.C. 1973. The biological efficiency of protein production by stallfed ruminants, p. 237-262. In J.G.W. Jones, ed. The biological efficiency of protein production. Cambridge, UK, Cambridge University Press.
Little, D.A., Anderson, F.M. & Durkin, J.W. 1991. Influence of partial suckling of crossbred dairy cows on milk offtake and calf growth in the Ethiopian Highlands. Trop. Anim. Health Prod., 23(2): 108-114.
Trail, J.C.M. & Gregory, K.E. 1981. Sahiwal cattle: an evaluation of their potential contribution to milk and beef production in Africa. ILCA Monograph No. 3. Addis Ababa, Ethiopia, International Livestock Centre for Africa. 127 pp.
Watson, D.M.S. 1943. Beef cattle in peace and war. Emp. J. Exp. Agric., 11: 191-228.
