The project will determine the extent of mobilization/retention under different energy supply conditions over the course of an entire lactation cycle.

At the beginning of lactation (the phase of highest nutrient requirements and lowest feed intake), the cow mobilizes nutrients (especially fat reserves) to meet the demands of milk synthesis. The second third of lactation is characterized by a relatively balanced energy balance. In the final phase of lactation, and particularly during late pregnancy, nutrient retention (replenishment) occurs, which is then available for mobilization at the beginning of the next lactation, and so on. The extent of mobilization depends on the energy supply (concentrate content, forage quality) and the cow's milk production potential (POND 1984, CHILLIARD 1987, CHILLIARD et al. 1991, CHILLIARD et al. 2000, BELL & BAUMAN 1994, McNAMARA 1994, SCHWAGERSUTER et al. 2001).

This production cycle of mobilization and retention, found in all female mammals, has both positive and negative aspects. From an evolutionary perspective, it was particularly advantageous for the survival rate of the offspring that their nutrition via breast milk was (partially) independent of the mother's nutrient supply (due to her ability to mobilize). This genetically and evolutionarily deeply rooted ability to mobilize can also be used to advantage under modern milk production conditions by ensuring that the nutrients required for milk synthesis do not have to be entirely covered by feed, but can be partially obtained through mobilization. As a result, rations with a lower concentrate content are possible (reduced rumen load [acidosis, ruminant-friendly diet, etc.], more species-appropriate, i.e., forage-based, rations, ecological and economic aspects [purchased inputs, imported nutrients]). However, this is countered by the fact that the mobilization of fat produces metabolites (β-hydroxybutyric acid, acetone, acet-acetate) that are highly taxing or damaging to the metabolism and cause the economically very important disease of ketosis (acetonemia) (severe decline in performance, frequent loss of animals).

This project investigates the extent of mobilization/retention under varying energy supply conditions throughout an entire lactation cycle. This is done directly in respiration chambers (parallel experiment at the University of Hohenheim) or through indirect or estimation methods (D₂O, 12C/13C isotopes; Body Condition Score). Simultaneously, the effects on metabolism and hoof health are precisely monitored. The results will allow for the derivation of the optimal energy level in dairy cattle feeding, thus addressing a central aspect of dairy cattle nutrition (EDMONSON et al. 1989, GEARHART & CURTIS 1990, RUEGG & MILTON 1995, GILLUND et al. 2001, PRYCE et al. 2001). The research question leads to the following experimental design:

Experimental design: Concentrated feed proportions during lactation phases

For this experiment, 63 cows from the dairy herd of the LFZ Raumberg-Gumpenstein are being used. The experimental design specifies that the different energy supply levels in the individual lactation phases are achieved through varying concentrate feed proportions. Feeding at the end of lactation and during the dry period is identical for all groups in order to determine the influence of the energy supply at the beginning of lactation on the response of all experimental parameters (feed intake, nutrient retention, metabolism, etc.).