**6. Maternal component of growth**

Growth traits like birth and weaning weights are determined by the animal's own additive genetic merit as well as the maternal component, which can be further separated in an additive genetic and a permanent environmental component [51]. The maternal component mainly represents the dam's milk production and mothering ability, although the uterine environment and extra-chromosomal inheritance may also have an effect. The dam's genotype therefore has an effect on the phenotype of the young through a sample of half her direct, additive genes for growth as well as through her genotype for maternal effects on growth [52].

status and parity of the cow, suckling interval (see [40, 44]), cow-calving season due to nutritional factors and or light and temperature and dystocia (see [44, 45]), the presence of a bull (see [45]) breed and age of parity also has an influence (see [45, 46]) as well as sire breed.

Although many factors affect postpartum anoestrus, nutrition and suckling are the major influences on the resumption of postpartum ovarian cycles, as it affect hypothalamic, pituitary and ovarian activity and therefore inhibit follicular development [47, 48]. Under-nutrition contributes to prolonged postpartum anoestrus, particularly among cows dependent upon forage to meet their food requirements [40]. The nutritional status or balance of an animal is evaluated by means of the Body Condition Score (BCS) parameter. BCS reflects the body energy reserves available for metabolism, growth, lactation and activity. There is a relationship between energy balance and time to the resumption of postpartum ovarian activity.

Inadequate nutrition cause excessive weight loss, followed by a decrease in BCS and finally cessation of the oestrus cycle. Suckling probably interferes with the hypothalamic release of GnRH and suppresses the pulsatile release of LH which leads to an extended postpartum anoestrus [40]. However, the exact interaction by which suckling extends post-partum anoestrus is uncertain [49]. The huge benefits of 12- or 48 hour calf removal prior to the onset of breeding were clearly demonstrated [47, 48]. This research disclosed significant improvements in conception rates from 55 to 76% in Brahman-type cows in a semi-arid environment. Such strategies are beneficial in terms of beef cattle production, but are not widely employed in Southern Africa. Other factors that influence the anoestrus period after calving and cause a longer inter-calving period are: general infertility, uterine involution, short oestrus cycles and post-partum anoestrus [45]. Management practices play an important role in the ICP of a herd and the following

• Use a sterile teaser bull with cows during the early postpartum period before the breeding

Although there are numerous objections to the use of inter calving period (ICP) as a measure of female reproductive performance, there is no alternative to ICP as a measure of reproduc-

Growth traits like birth and weaning weights are determined by the animal's own additive genetic merit as well as the maternal component, which can be further separated in an

practices may decrease PPA:

• Minimize dystocia distress.

• Decrease suckling stimulus.

**6. Maternal component of growth**

season starts.

• Synchronize oestrus.

tive performance [50].

• Introduce a short breeding season.

50 Ruminants - The Husbandry, Economic and Health Aspects

• Make use of BCS to monitor nutritional management.

Postnatal calf growth and physiological development are initially influenced by stimuli experienced *in utero* [53]. Maternal nutrition therefore potentially affects not only cow productivity but also post-weaning calf productivity [54]. Protein supplementation during late gestation, as well as increased total nutrient supply throughout gestation, may increase calf birth weight [54]. Another major component of the maternal environment created by the dam is the nutrition the calf receives through milk. There is a positive relationship between the breeding value for milk for the dam, actual milk production and the weaning weight of calves [55]. A high correlation (0.8) was reported between direct milk yield and maternal weaning gain (see [56]).

Milk intake also influences forage intake of nursing calves, e.g., calves of dams with lower milk production are more reliant earlier in lactation, on alternative feed sources of lower nutritional value than milk [57]. Calf body weight and forage dry matter intake are correlated with calf milk intake, and nursing calves generally become increasingly dependent on forage after 60–90 days of age to maintain normal growth. It follows that the forage quality of rangeland systems affect growth rates of calves through influences on the milk yield of dams and quality of the forage portion of a calf's diet.
