Over the last few decades, the use of the pure Holstein breed and intense genetic selection have significantly increased milk yields of dairy cattle and, therefore, production efficiency of the dairy industry. Unfortunately, it has now become evident that this increase in milk yield is accompanied by some adverse consequences, such as an increase in the incidence of metabolic diseases, lameness and mastitis, and a reduction in reproductive performance (Pryce and Lovendahl, 1999; Ward and Parker, 1999; Webb et al., 1999a). This finding has given rise to the concern that the high yielding dairy cow may be under a state of metabolic stress during peak lactation and, therefore, the welfare and performance of other body functions are compromised (FAWC, 1997; Nielsen, 1999). Observations have shown that reproductive performance has been decreasing in the high yielding dairy cow, especially when the animals are under severe negative energy balance (Butler and Smith, 1989; Nebel and McGilliard, 1993; Ouweltjes et al., 1996; Snijders et al., 2001). For example, intervals from calving to the first ovulation post partum and to conception have been observed to be longer in lactating cows with a higher milk yield than in cows with a low milk yield (Beam and Butler, 1999). Recent data have indicated that conception rate to first service has been declining by approximately 1.0 and 0.5% per annum in the UK (Royal et al., 2000) and the USA (Beam and Butler, 1999), respectively, over the last 25 years. Although this decline has been attributed, at least in part, to intense genetic selection based mainly on production traits, such as milk yield (Pryce et al., 1997;