**5.1. Inter-calving period**

accepted that in South Africa the calf crop averages between 60 and 65% per annum [27]. Conception rates of cow herds are influenced by a number of interacting factors such as (a) plane of nutrition of bulls and cows, (b) the age of the breeding animals, (c) herd health, (d) libido and (e) semen quality of bulls as well as (f) the ability of cows to conceive and

The reproductive ability of a cow is determined by her performance in terms of a number of different reproductive functions that occur throughout her lifecycle. These functions can be divided into component and aggregate traits. A component trait is a single event while aggregate traits are composites of more than one reproductive event [28]. Some of the component traits that can be measured, include time to first oestrus, number of services per conception, pregnancy rate, heifer pregnancy, gestation length, days to calving, age at first calving, calving date, calving ease, calving interval and days open. A combination of these traits are often used to form aggregate traits such as, calving rate, lifetime pregnancy rate, calving success, calf survival and lifetime production. Although these traits might reflect an indication of reproductive performance there are unfortunately no completely satisfactory measure/s of reproduction efficiency [30, 31]. This is due to the influence that the age structure of the herd as well as the prevailing environmental and management conditions have on reproductive recording [28]. Traits that are most frequently used to evaluate reproductive performance are AFC and ICP as well as Reproduction Index (RI), and post-partum anoestrus remains one of the most limiting factors [22, 28, 29].

Age at first calving (AFC) is an important production parameter for commercial beef cattle producers, since it affects the size of cows as well as weight and number of calves produced. AFC also affects the potential annual genetic progress for stud farmers [32]. Beef heifers are generally managed to calve for the first time at either 2 or 3 years of age [32, 33]. Mating heifers earlier may increase dystocia and there are conflicting reports on the lifetime production span of early mated heifers. Some authors reported an increase in the number of calves and weaned kilograms (see [32, 34]), while others reported no increase in the weaned weight, despite the birth of an extra calf [33]. Nevertheless, the success of mating heifers at a younger age depends on nutritional and management levels (see [33]), and most heifers have

There is a great deal of controversy regarding the use of AFC as a measure of female reproductive ability in the literature. The biggest advantage of AFC is that it can be easily recorded because the birth date of the cow and its first calving date are generally known, while the main disadvantage is that it only represents a single component in the reproductive life of a cow [28]. The general consensus is that in a variable seasonal environment, management decisions often have a greater effect on AFC than genetic merit. Researchers increasingly question the use of AFC, because AFC and the probability of heifers to reconceive are determined by

It was argued by several that reducing the AFC is one of only a few means of improving lifetime production efficiency in the beef cow herd [33, 34]. Shorter AFC values naturally reduce the generation interval, and thus contribute to the annual genetic gain of the herd [36]. Another common but erroneous belief is that scrotal circumference in yearling bulls may be an indicator of reproductive fitness in female offspring [35, 37]. Scrotal circumference was

the potential to reach puberty and breed satisfactorily in such systems [35].

maintain pregnancy [28, 29].

48 Ruminants - The Husbandry, Economic and Health Aspects

different genes.

ICP or calving interval is an aggregate reproductive trait, composed of more than one reproduction event, and is defined as the time that elapsed between two successful calving's [28]. ICP is regarded as an important fertility trait, especially if one considers the importance of reproduction in a calf production system [39]. The ideal would be that every cow should calf every year and that the ICP of a beef cattle herd should be less than 365 days [38, 40]. This means that a cow should conceive within at least 80–90 days after calving, but it is accepted that the ICP in many breeding herds often exceeds 365 days in the tropical or subtropical areas due to high humidity and temperature and lower forage quality [41]. According to the SANBRIS, the current ICP average for the different breeds in Southern Africa ranges between 398 and 477 days. The Hereford and Shorthorn breeds have the shortest (398 days) and the Huguenot the longest (477 days) ICP, while the average ICP of the Bonsmara breed is 405 days [27].

The use of ICP as a measure of reproductive efficiency in a fixed breeding season has been questioned by several authors [28, 30, 42]. The major criticism against ICP as a selection criterion for reproductive performance is the negative correlation that exists between ICP and previous calving date as well as the large influence that the previous calving date has on the ICP [42]. This means that cows that calve early in the season have the longest ICP while those that calve late in the season have the shortest ICP.

The low heritability of ICP is also another question raised. The estimated heritability ranges between 0.02 (see [43]) and 0.12 (see [31]), with a low repeatability of 0.14 [43]. The repeatability estimate for ICP suggests that female culling based on first calving interval is not accurate and there is a risk of culling animals with other desired traits. Selection for shorter ICP's could result in indirect selection for a later age of puberty as cows with the shortest calving interval, are often those who calved late in the season [30]. It also does not take information from the first parity or the end of a cow's life span into account when the ICP of the herd is determined [28].

The analysis of ICP is also problematic because it is only available for cows that calve repeatedly and should therefore be treated as a censored trait. Fortunately ICP is based on the period between two calving's; it can therefore be easily computed with a minimum of data, and this data will be lost from the reproductive information for the first parity as well at the end of a cow's lifespan if no calf is born [28, 29].
