Impacts of feeding intensity and breed on metabolism, negative energy balance and reproductive efficiency in dairy cows

Despite improved breeding, management, and nutritional strategies, decreased fertility in dairy cows is still widespread. Several studies have highlighted the unfavourable correlation between negative energy balance (NEB) and reproductive performance. However, there is a continuing need for more information regarding the effects of the interaction between different nutritional strategies with animals of different genetic background. This thesis evaluated the effect of high energy-diet (HE) and low energy-diet (LE) on Holstein and SRB dairy cows. The HE represents what is, in general, common practice among most high milk producing herds. The lower feeding intensity may be representative of e.g. organic dairy producing systems. In three studies, the metabolic status, milk yield, body condition score (BCS), and NEB were evaluated in cows fed either a HE or a LE diet. Associations between NEB, plasma adipokines, metabolism, and reproductive parameters were also investigated. Holstein cows had lower body condition score (BCS) than SRB cows within each energy-diet group. However, diet had no effect on BCS or subcutaneous adipose tissue thickness in Holstein cows irrespective of whether they received a HE or LE diet. The HE group tended to have a less severe energy deficit than the LE group. Holstein cows tended to be in a more severe energy deficit during the first 45 days after calving than SRB cows. Holstein had a lower nadir in energy balance than the SRB. In conclusion, our results indicate that nutritional strategies might have a stronger association to endocrine and traditional fertility traits than breed. However, breed had a stronger association to the energy balance variables than nutritional strategies. In addition, SRB cows prioritized energy differently when compared to Holstein cows in such a way that the SRB cows maintained homeostasis better than Holsteins who had a deeper energy deficit than SRB cows.