The length of time dairy heifers survive in a herd and longevity of dairy cattle, was found to decrease significantly between 1981 and 1992, with the average herd life of a dairy cow in the 1990's declined to 33 (± 0.38) months or 2.75 years (Durr, Monardes, Cue and Philpot, 1997). In the UK, 21 % of dairy heifers are culled during the first lactation (MDC, 1998) while in the EU 33.4 % of heifers complete only one lactation (Durr et al., 1997). In the UK dairy herd 48 % of heifers complete three lactations (MDC, 2000), while in the EU as a whole only 17 % of heifers remain in the herd for their third lactation (Durr et al., 1997).

The high yielding dairy cow is expected to produce a substantial milk output every year and at the same time to conceive and maintain a pregnancy to term. To fulfil lifetime production potential a balance between yield, fertility and other influential factors has to be achieved. Any inability on the part of the management system to identify and rectify problems or on the part of the cow to cope with metabolic demands invariably results in economic or welfare issues. Our studies of high yielding dairy cows have revealed that some animals are capable of normal reproductive function whilst others are classic repeat breeders (requiring 3+ services per conception) or simply fail to rebreed. It is well established that the somatotrophic axis (growth hormone and insulin-like growth factors) drives lactation in ruminants but it is also intimately involved in reproductive processes. An awareness of metabolic adaptations to lactation that impact on reproduction in dairy cows is needed for appropriate management.

The objective of our studies was to explore the metabolic profiles of high yielding dairy cows to identify factors influencing their ovarian function and fertility, hence to characterise the physiological adaptations involved. Our studies revealed different relationships between progesterone profile categories and metabolic status post partum. Delayed ovulation (DOV) or persistent corpora lutea (PCL) may be an appropriate response to a nutritional state or physiological situation and it may therefore be inaccurate to refer to these as ‘abnormal’. Whilst associated with high milk yields, not all profile categories detrimentally affected fertility parameters. Delayed ovulation postcalving (DOV1) was identified as the most prevalent abnormal profile encountered in first lactation high yielding cows. This may have occurred because the cows were not yet physically mature and unable to sustain both milk production and growth. The condition lasted long enough (71 ± 8.3 days from calving) to have a detrimental impact on their overall fertility parameters and was associated with significant physiological changes, representative of tissue mobilisation. Although the incidence of persistent luteal phases (PCL1 and PCL2) in dairy cows is increasing, this condition was not found to have any substantial detrimental effects on fertility or production parameters of the primiparous or multiparous cows in these studies. The main reproductive problems in our high yielding primiparous and multiparous cows appeared to be a failure to ovulate and conceive at the expected time or to maintain a pregnancy. These situations were associated predominantly with high milk yields and low concentrations of plasma IGF-I. A failure to ovulate appears to occur when body reserves are mobilised to maintain milk yield at the expense of reproduction and seems most likely to occur in primiparous high yielding cows or those experiencing GH-resistance (low IGF-I) due to excessive body condition loss, reduced feed intakes and factors such as stress and disease. More detailed investigations of dietary means of increasing IGF-I and optimising insulin concentrations, targeted at important reproductive times, are required in high yielding dairy cows, to aid in their management.

Nearly 50 years ago, a cow named Lorna achieved notoriety by producing nearly 50 kg of milk daily during measurements of her energy metabolism in a calorimeter at the United States Department of Agriculture (USDA) Energy Metabolism Laboratory at Beltsville, Maryland (Flatt, Moore, Hooven and Plowman, 1965; Flatt, Moe, Munson and Cooper, 1969). In the intervening period genetic selection in dairy cattle has produced huge increases in average milk yield, as well as changes in overall conformation, udder characteristics, and body size and structure (Hansen, 2000).

Investigations of culling and particularly involuntary culling (“functional” longevity) have often used different criteria on herds of varying size in different regions and with divergent average milk outputs (Young, Waddington, Sales, Bradley and Spooner, 1983; Esslemont and Kossaibati, 1997; Forbes, Gayton and McGeogh, 1999; Whitaker Kelly and Smith, 2000; Figure 1). Thus the evidence of increasing or decreasing culling for any particular entity is uncertain.

During recent decades, the UK dairy industry has had to adjust to the introduction of milk quotas in 1984, the deregulation of milk markets in 1994, and accommodate changes in the demand for dairy products. The combination of these factors, in addition to Bovine Spongiform Encephalopathy and Foot and Mouth disease, and a fall in milk price has inevitably resulted in a restructuring of the industry, but also reinforced the need for all sectors of the industry to respond to the prevailing economic climate and changes in consumer preferences.

The human race has previously utilized biotechnology (defined as the use of microorganisms or biological substances to perform industrial purposes) for thousands of years to produce foods such as beer, wine, cheese and yogurt. In recent decades, biotechnology has been successfully employed to improve plant and animal products beneficial to livestock and humans while indirectly improving the environment through pesticide reduction.

Chronic diet related diseases such as cardiovascular disease (CVD) and cancers account for the vast majority (∼ 65%) of total mortality in the UK (Figure 1). In westernised countries average life expectancy is rapidly increasing with the ratio of people of working age to people over 65 estimated to fall from about to 4:1 to 2.5:1 by the year 2040. These ageing population demographics have placed an almost unbearable strain on the health care systems of these countries. As a result there has been increased focus on the use of diet as a modifiable means of preventing or delaying the onset of disease. This approach in addition to being cost effective would ensure that for the individual who is living longer that they also remain healthier for longer, developing chronic life threatening diseases at an older age.