1. Introduction
Growth in animals is defined as accretion of protein, fat and bone in the body. Although growth typically is measured as the change in live weight, nutrient retention is estimated more precisely by measuring empty body weight and its composition. Production economics are measured ideally through carcass weight and carcass quality. As a percentage of live weight gain, carcass weight gain usually is a much higher percentage during the feedlot phase than during the growing phase of production because dressing percentage increases with maturation and is greater with concentrate than with roughage diets. At a given fraction of mature body size, body fat percentage seems to be constant. Mature size may be altered genetically and nutritionally. Protein accretion declines to zero when cattle reach their mature body size. Although fat concentration can be reduced by limiting the supply of net energy, rate of fat accretion by finishing steers given ad libitum access to high-concentrate diets seems to reach a plateau at approximately 550 g daily. Protein mass, in contrast, increases in proportion to empty body weight. The protein:fat ratio of the carcass can be increased through increasing mature size, by administering hormones, by limiting energy intake during the growing period or finishing period, or by slaughtering cattle at an earlier stage of maturity. Energetically, efficiency of accretion of fat is approximately 1.7 times that of protein. But because more water is stored with deposited protein than with deposited fat, lean tissue gain is four times as efficient as accumulation of fat tissue.
2. Importance of growth in livestock
From birth until weaning to dry feed, the calf undergoes tremendous physiologic and metabolic changes. During the pre ruminant stage, digestion and metabolism are similar to those of non ruminant animals in many respects. Thus, dietary requirements are best met with high-quality liquid diets formulated from sources of carbohydrates, proteins and fats that are digested efficiently. The most critical period is the first 2-3 week of life, during which the calf’s digestive system is immature but developing rapidly with regard to digestive secretions and enzymatic activity. Calves raised for purposes other than veal production should be encouraged to consume dry feed at an early age so as to stimulate development of functional rumen. Development of rumen epithelial tissue that is responsible for absorption of volatile fatty acids (VFA) depends on presence of VFA, particularly butyrate.
3. Average daily gains of some beef breeds
| Name of beef breed | Average daily gain (lb) | Mature body size (lb) |
| Orig. Hereford-Angus | 2.51 | 1068 |
| Curr. Hereford-Angus | 2.74 | 1152 |
| Orig. Charolais | 2.77 | 1160 |
| Curr. Charolais | 2.89 | 1219 |
| Chianina | 2.63 | 1124 |
| Gelbvieh | 2.66 | 1129 |
| Limousin | 2.49 | 1080 |
| Maine Anjou | 2.72 | 1147 |
| Salers | 2.70 | 1148 |
| Shorthorn | 2.73 | 1156 |
| Simmenta | 2.73 | 1148 |
4. Role of energy, protein, vitamins and minerals
Energy requirements of calves, like those of other ages and classes of cattle can be expressed in numerous ways. Regardless of the system preferred, it is crucial to understand where the major losses of energy occur as energy-yielding components of the diet undergo digestion and metabolism. If the efficiencies of conversion of gross energy to digestible energy or metabolizable energy and of conversion of metabolizable energy to net energy are known, users can select the system that best fits their needs. The energy requirements of calves have been derived on the basis of metabolizable energy; however, requirements and feed composition also are given in units of net energy and digestible energy for those who prefer to use those systems. Data on energy requirements are organized around replacement calves fed only milk or milk replacer, calves fed milk and starter feed or milk replacer and starter feed; calves reared for veal on only milk or milk replacers and weaned replacement calves to 100 kg of body weight fed starter or grower diets .The amount of liquid feed (milk or milk replacer) offered to replacement calves is restricted to encourage intake of dry feed (starter), but calves reared for veal are fed milk or milk replacer at near ad libitum intakes. Energy is considered first in diet formulation. Energy intake can determine the ability of the calves to utilize other nutrients to a large extent. Energy requirements are affected by mature body size (higher maintenance needs for larger calves), physical activity (30 to 50% more free-grazing vs. confined calves), and environmental temperatures.
There is little information on the protein requirements of young calves weighing less than 100 kg. The requirement is partitioned into components of maintenance and gain. Maintenance constitutes obligatory nitrogen (N) losses in urine and feces, whereas gain pertains to N stored in tissues. Microbial protein supplies about 50% of protein and amino acid needs. Protein deficiency is most common deficiency in calves grazing mature forages or low-quality hays, or straw. Less than 7% CP is not enough to provide sufficient ruminal N/ammonia for microbes, resulting in reduced digestion of forages & forage intake.
In case of growing animals, most important role is played by calcium and phosphorus. If not consuming adequate amounts, various deficiency disorders can develop, e.g., rickets in young animals & osteomalacia in adults. Cobalt is needed by microbes to synthesize vitamin B12. Where iodine is low in the soil, the use of iodized salt would be the most convenient way to provide I2. Forages are relatively high in many minerals (e.g., Cu, Fe, Mg, Mn,etc.), but grains are usually very low in many minerals.
Vitamins-Sun-cured forages contain large amounts of vitamin D, vitamin E, and β- carotene. Vitamin A can be stored enough for 2 to 4 months., but would be depleted quickly without dietary Vitamin A. Once the calf is weaned to dry feed, there is no evidence that water soluble vitamins need to be supplemented in diet, as much as the micro organisms in the digestive tract synthesize ample amounts to meet the needs of calf.
Conclusions
Growth rate can be defined mathematically in terms of mature size, the rate of growth deceleration and age. Although maximum growth rate is set genetically, nutritional and hormonal factors can limit growth rate and, by altering mature size, may reduce or increase fat content of the carcass at a given weight. Heavy mature weights are not desirable for ruminants maintained for reproduction. Growth stimulants or depressants and agonists that alter body size and composition may prove as useful to enhance production efficiency in the cow herd as in finishing animals.
Neeti Lakhani1 and Preeti Lakhani2
1. College of Veterinary Science, Rampura Phul, GADVASU, Bathinda, Punjab, 151103
2. College of Veterinary Science, LUVAS, Hisar, Haryana, 125001