The main objective of the dairy based production systems is to produce milk as economically as possible (Lucy, 2001; Dobson et al., 2007). In intensive dairying systems animal breeding aims to maximize milk yield, although milk composition and other factors like fertility and health are becoming increasingly important (Pryce et al, 1998; Ball & Peters, 2004; Oltenacu & Algers, 2005). In such production systems, most male calves are a by-product and are sold or reared for beef production. Semen of bulls selected as high genetic merit sires are used in artificial insemination (AI) schemes aiming to transfer the genetic progress in the herd (Rook & Thomas, 1983; Ball & Peters, 2004). The increased milk production have been mostly achieved due to steady genetic progress in the commercial herds based on continuous importation of genes from USA and Canada and widespread use of high genetic merit sires in AI schemes (Garnsworthy & Webb, 1999) .
Milk yield and milk composition are related to genetic, nutritional and environmental factors (Rook & Thomas, 1983). On a short-term basis, the efficiency of nutrient use for milk production is primarily dependent on the milk production level. As milk yield increases, a lower proportion of total feed intake is used for maintenance of the cow (Chilliard, 1992). In generally, in high yielding dairy cows milk production and reproductive performance are highly negatively correlated (Macmillan et al., 1996; Royal et al., 2000; Stevenson, 2001; Opsomer et al., 2006).
The last decades, the genetic improvement in dairy cows has led to a dramatic increase in milk yield, which has been tied up with an inherent decline in reproductive performance (Butler, 2000; Royal et al., 2000; Evans et al., 2006; Garnsworthy et al., 2008a). Probably, many other reasons are at least in the same degree accountable for this decrease in reproduction performance such as nutrition, management and poor expression or detection of estrus (Garnsworthy & Webb, 1999; Lucy, 2001; Dobson et al., 2007).
On the whole, nutritional factors interact with reproductive performance with prominent impacts on livestock productivity and viability (Butler and Smith, 1989; O'Callaghan and Boland, 1999; Garnsworthy & Webb, 1999). At present, these interactions are yet on study but it seems that the relationship between nutrition and reproduction is dynamic, complex, not well understood and responses are often quite variable and inconsistent (Gordon 1996; Boland & Lonergan, 2003). The reproductive efficiency of the postpartum cow is a critical element determining overall biological and productive efficiency. Thus, failure of cows to resume cyclicity after calving is a critical point that in general influencing the economical profitability of the cattle industry (Roberts et al., 1997; Stott et al., 1999; Evans et al, 2006; Garnsworthy et al., 2008a), especially in the intensive milk producing systems.
In general, high producing dairy cows experiences a metabolic stress involving NEB in early lactation (Bauman & Currie, 1980). The main characteristics of this hard condition are loss of body weight and mobilization of body fat stores because feed intake cannot support the energy required for milk yield and maintenance (Beam & Butler, 1999; Butler, 2000; De Vries & Veerkamp, 2000). Energy requirements to support follicle growth and ovulation are negligible (less than 3 MJ ME/ day) compared with requirements for maintenance and production (60-250 MJ ME / day in a lactating dairy cow). However, in the case of lactating dairy cows, inadequate nutrition in the short term, or as a consequence of a prolonged depletion of body reserves during early lactation, can have significant detrimental effects on resumption of ovarian activity postpartum, conception rate and infertility (Boland & Lonergan, 2003).
在一般情况下，高产奶牛经验的代谢应激涉及NEB在泌乳早期（鲍曼和库里，1980）。这种硬条件的主要特点是体重和体脂储备动员损失因为采食量不能支持牛奶产量和维护所需的能量（梁管家，1999；Butler，2000；de Vries和Veerkamp，2000）。能源需求以支持卵泡发育及排卵是微不足道的（小于3 MJ我/天）与维修和生产要求相比（在泌乳奶牛60-250 MJ我/天）。然而，在泌乳奶牛的情况下，在短期内营养不足，或泌乳早期长期消耗体内储备的结果，可以对产后恢复卵巢活动显著不利影响，受孕率和不育（2003公司纳根）。
体况评分（BCS）一直被认为是营养状况的一种间接测量（麦克米兰et al.，1996；加恩兹沃西，2006）。众所周知，奶牛的身体状况，特别是在其年度生产周期直接影响生育的某些点（巴特勒，2003；加恩兹沃西，2006）。这是真正的各种重要的肥力指标其中产犊间隔（CI）（露西2001；oltenacu与阿尔及利亚，2005；Dobson等，2007）。产犊时的身体状况被认为通过对早期泌乳及子宫复旧的组织动员影响其繁殖性能。此外，BCS是条件损失和能量负平衡（NEB）密切相关，从而影响循环代谢激素和代谢物（2003公司纳根）。最近的报告表明，平均目标BCS现在比它更低的年代。这是因为选择较高的牛奶产量在过去的三十年里，牛的基因更薄，所以他们倾向于动员体内脂肪。因此，高遗传价值的牛会在泌乳早期和更长期的负能量平衡更深的（加恩兹沃西，2006）。
