The rearing period has a key influence on the later performance of cattle, affecting future fertility and longevity. Producers usually aim to breed replacement heifers by 15 months to calve at 24 months. An age at first calving (AFC) close to 2 years (23 to 25 months) is optimum for economic performance as it minimises the non-productive period and maintains a seasonal calving pattern. This is rarely achieved in either dairy or beef herds, with average AFC for dairy herds usually between 26 and 30 months. Maintaining a low AFC requires good heifer management with adequate growth to ensure an appropriate BW and frame size at calving. Puberty should occur at least 6 weeks before the target breeding age to enable animals to undergo oestrous cycles before mating. Cattle reach puberty at a fairly consistent, but breed-dependent, proportion of mature BW. Heifer fertility is a critical component of AFC. In US Holsteins the conception rate peaked at 57% at 15 to 16 months, declining in older heifers. Wide variations in growth rates on the same farm often lead to some animals having delayed first breeding and/or conception. Oestrous synchronisation regimes and sexed semen can both be used but unless heifers have been previously well-managed the success rates may be unacceptably low. Altering the nutritional input above or below those needed for maintenance at any stage from birth to first calving clearly alters the average daily gain (ADG) in weight. In general an ADG of around 0.75 kg/day seems optimal for dairy heifers, with lower rates delaying puberty and AFC. There is some scope to vary ADG at different ages providing animals reach an adequate size by calving. Major periods of nutritional deficiency and/or severe calfhood disease will, however, compromise development with long-term adverse consequences. Infectious disease can also cause pregnancy loss/abortion. First lactation milk yield may be slightly lower in younger calving cows but lifetime production is higher as such animals usually have good fertility and survive longer. There is now extensive evidence that as long as the AFC is > 23 months then future performance is not adversely influenced. On the other hand, delayed first calving > 30 months is associated with poor survival. Underfeeding of young heifers reduces their milk production potential and is a greater problem than overfeeding. Farmers are more likely to meet the optimum AFC target of 23 to 25 months if they monitor growth rates and adjust feed accordingly.

Purebred Holstein-Friesian cows are the main exotic breed used for milk production on large, medium, and small farms in Kenya. A study was undertaken on seven large-scale farms to investigate the genetic trends for milk production and fertility traits between 1986 and 1997 and the genetic relationships between the traits. This involved 3,185 records from 1,614 cows, the daughters of 253 sires. There was a positive trend in breeding value for 305-d milk yield of 12.9 kg/ yr and a drop in calving interval of 0.9 d/yr over the 11-yr period. Bulls from the United States (U.S.) had an average total milk yield breeding value 230 kg higher than the mean of all bulls used; Canada (+121 kg), Holland (+15 kg), the United Kingdom (U.K., 0 kg), and Kenya (-71 kg) were the other major suppliers of bulls. Average breeding values of bulls for calving interval by country of origin were -1.31 (Canada), -1.27 (Holland), -0.83 (U.S.), -0.63 (Kenya), and 0.68 d (U.K.). The genetic parameters for 305-d milk yield were 0.29 (heritability), 0.05 (permanent environment effect as proportion of phenotypic variance) resulting in an estimated repeatability of 0.34. Using complete lactation data rather than 305-d milk yield resulted in similar estimates of the genetic parameters. However, when lactation length was used as a covariate heritability was reduced to 0.25 and the permanent environment effect proportion increased to 0.09. There was little genetic control of either lactation length (heritability, 0.09) or calving interval (heritability, 0.05); however, there were strong genetic correlations between first lactation milk yield, calving interval, and age at first calving.

Abstract. Dairy cows mobilise body tissues to support milk production and, because glucose supplies are limited, lipids are used preferentially for energy production. Lipogenic activity is switched off and lipolytic mechanisms in adipose tissue increase through changes in the expression of several key enzymes. This results in a loss of body condition, together with high circulating concentrations of non-esterified fatty acids. Changes in the synthesis, secretion and signalling pathways of somatotrophic hormones (insulin, growth hormone, insulin-like growth factor 1) and adipokines (e.g. leptin) are central to the regulation of these processes. A high reliance on fatty acids as an energy source in the peripartum period causes oxidative damage to mitochondria in metabolically active tissues, including the liver and reproductive tract. The expression of genes involved in insulin resistance (PDK4, AHSG) is increased, together with expression of TIEG1, a transcription factor that can induce apoptosis via the mitochondrial pathway. Polymorphisms in TFAM and UCP2, two autosomal mitochondrial genes, have been associated with longevity in dairy cows. Polymorphisms in many other genes that affect lipid metabolism also show some associations with fertility traits. These include DGAT1, SCD1, DECR1, CRH, CBFA2T1, GH, LEP and NPY. Excess lipid accumulation in oocytes and the regenerating endometrium reduces fertility via reductions in embryo survival and increased inflammatory changes, respectively.

The recent decline in dairy cow fertility appears to be a feature of several countries and is often linked to increased milk production, but its causes are not always obvious. A fully recorded 200-cow dairy herd, split into 2 genetic lines maintained on 2 production systems, was used to investigate the relationship between several measures of fertility, production, and energy balance. The 2 genetic lines were composed of a selection line, derived from the highest genetic merit bulls available, and a control line, maintained at the average of UK genetic merit at the time of mating. The production systems were a high-concentrate and a high-forage system. Thrice-weekly milk progesterone samples allowed an objective measurement of luteal cycling activity, and farm observations of estrus, services, and calving provided data on various measures of fertility. Energy balance in early lactation was calculated from daily live weight and weekly BCS measurements. Control line cows commenced luteal activity (C-LA) 6 d before selection line cows, had their first heat 14 d earlier, and had longer gestation periods by 3.7 d. They also had a lower incidence of silent heats. Cows on the high-forage system commenced luteal activity 6 d before those on the high-concentrate system, had longer gestation intervals by 3.9 d, held to first service better, had longer luteal phases and shorter interluteal periods in their estrus cycles. Characteristics of energy balance were used to see if they could account for the fertility differences between both genetic lines and systems. The commencement of luteal activity and day of first heat were analyzed using a REML mixed model approach. Mean energy content and mean energy balance over the first 25 d of lactation had an effect on C-LA and accounted for the differences found between production systems but not genetic lines. Day of energy balance nadir, mean energy content in the first 25 d, and C-LA affected day of first heat, but the differences between genetic lines were still apparent. These results suggest a link between high performance and reduced dairy cow fertility; high performance originating from different feeding systems was largely due to differences in energy balance, whereas those originating from genetics remained when energy balance characteristics were taken into account. This suggests a real genetic change in fertility due to selection for high genetic merit.

Breeding for host resistance to parasites has become an imperative in many sheep industries. Because of the widespread use of AI in sheep breeding schemes, it is important to understand how the performance of offspring from rams varies in different flock environments, both for resistance to parasites and key production traits. This study used both variance component and reaction norm models to investigate the level of genotype x environment interaction for fecal egg count (FEC) and important Merino production traits in a range of flock environments in Australia. These flocks were linked by the use of common rams in a sire-referencing scheme. Both linear and quadratic polynomial reaction norm models were used. The heritability of these traits and the genetic correlation between them and FEC also was investigated using the reaction norm model. A contemporary group (CG) was defined by a flock, year, age class, sex, and paddock combination. Each CG environment was characterized by the mean value of any given trait for that CG. The recorded data used in the study were analyzed in a standardized form. Standardization for each trait was achieved within a CG by subtracting the CG mean from each observation and dividing by the CG SD. The genotype x environment effect accounted for <0.05 of the phenotypic variance for all traits. In most traits the heritability varied little across environments. The exceptions were FEC, BW, and both greasy and clean fleece weights, which had a higher heritability at the lower end of the environmental range. Fecal egg count also had a higher heritability in high-FEC environments. Genetic correlations between FEC and several key production traits were similar in the flock environments studied. Quadratic polynomial models and models with a variable residual fitted the data better than linear models. The genotype x environment effect for FEC and the genetic correlations between FEC and production traits were effectively zero; thus, sheep breeding programs for increased parasite resistance can be run effectively by ignoring these factors. Some account should be taken of the high heritabilities of FEC and fleece and BW in different flock environments.