**2. Cow size and adaptability**

The environmental and genetic factors that influence mature cow size include nutrition and management functions, as well as climatic factors such as rainfall and temperature and temporary environmental effects such as differences in fill when weighed and other climatic factors may also influence mature weight [3]. Mature cow weight reflects differences in size associated with skeletal size and lean growth, as well as fatness [13]. The genetic proportion of mature cow weight is mostly due to additive genetic variation, but there are differences in opinion about exactly when cows reach mature weight, e.g., at either 4.5 years, 6.5 years as in Ref. [14], or about 7 years of age [15]. It is difficult to determine exactly when animals stop growing [16], but it is accepted that cows accumulate most of their final weight at 4-years of age and final height at 3-years of age [13].

Several authors have made suggestions about which mature size should be optimal for a particular environment. The significant influence of cow size on production efficiency is also the reason why traits such as mature weight, height and length, are included in selection criteria [13]. In the late seventies and eighties there was an international trend to select for larger cattle [17], resulting in a net increase in growth rate, but it had a negative impact on female fertility traits [18].

account for more than 40% of the total value of South Africa's agricultural output [1]. Only 15% of South Africa is suitable for arable farming, and more than 40% of the remaining 85% receive less than 375 mm rain per annum [2], which explains the relatively low agricultural production potential of the Southern African region. The South African National Strategic Plan for Agriculture endorses the fact that there is very little room for horizontal expansion of agriculture, due to environmental constraints [1], so increased agricultural production can only be achieved by improving the efficiency of production [3] and exploiting vertical integra-

Long term improvement of the efficiency of animal production can only be achieved through the identification and selection of genetically superior animals for breeding purposes [3, 6]. Selection can be done based on a combination of pedigree information, appearance, and performance recorded information and breeding values [7]. Beef cow efficiency will be highest when cow size is tailored to the environment and the animals are well adapted [8–10]. Cow size has an important influence on the way the cow responds to its production environment [11] and the adaptability of the animal [8]. Adapted animals are tolerant to adverse environmental conditions and are able to maintain reproduction efficiency [6]. In order to improve beef cow efficiency in Southern Africa it is therefore important to optimize cow size, adapt-

The adaptability of beef cattle in extensive production systems is critical and genetic gains in this regard can be best achieved by implementing some beef industry recommendations as

• Identification and characterization of the major beef cattle production environments, and their respective nutritional, physical, climatic, management and economic characteristics, • Defining the major physical, biotic, social and management stressors in each beef produc-

The environmental and genetic factors that influence mature cow size include nutrition and management functions, as well as climatic factors such as rainfall and temperature and temporary environmental effects such as differences in fill when weighed and other climatic factors may also influence mature weight [3]. Mature cow weight reflects differences in size associated with skeletal size and lean growth, as well as fatness [13]. The genetic proportion of mature cow weight is mostly due to additive genetic variation, but there are differences in opinion about exactly when cows reach mature weight, e.g., at either 4.5 years, 6.5 years as in Ref. [14], or about 7 years of age [15]. It is difficult to determine exactly when animals stop growing [16], but it is accepted that cows accumulate most of their final weight at 4-years of

Several authors have made suggestions about which mature size should be optimal for a particular environment. The significant influence of cow size on production efficiency is also the

tion in regions with a moderate or higher agricultural potential [4, 5].

ability and employ effective management practices.

42 Ruminants - The Husbandry, Economic and Health Aspects

listed in [12] namely:

tion environment.

**2. Cow size and adaptability**

age and final height at 3-years of age [13].

The maintenance overhead is one of the most important factors that determine the biological efficiency of beef cattle, for example an adult cow require more than 50% of her total energy intake for maintenance [11]. Kleiber's theory, however, states that metabolic weight = (live weight)0.75 [19]. Larger cows therefore consume more nutrients than smaller cows but the percentage additional nutrient requirement of larger cows are less than its additional weight as a percentage. For example, a cow with mature size of 545 kg weighs 20% more than a 454 kg cow, but its maintenance requirements are only 13% higher [20].

The results of [21] suggest that when nutrient availability is limited, breeds with a moderate genetic potential for growth and milk production are generally more efficient because of higher conception rates. Similar results were reported in [10] in extensively managed Santa Gertrudis cattle in a semi-arid environment. At high levels of nutrient availability breeds with the highest genetic potentials for growth and milk production are most efficient because feed availability is sufficient for the genetic potentials to be expressed. Cow efficiency is thus maximized at a level of feed intake that do not limit reproduction and also provides sufficient energy for milk production to meet the growth potential of the breed as expressed in the calf [21].
