Optimizing rumen functions in the close-up transition period and early lactation to drive dry matter intake and energy balance in cows.

The large deficit in energy intake in relation to energy requirements during the transition and early lactation periods means that high-producing cows need energy-dense rations. High-starch diets are intensively fermented by the microbial ecosystem in the rumen, giving rise to a high production of VFAs and resulting in a drop in pH and the accumulation of lactic acid, which exacerbates the decline in pH and is considered as the major cause of rumen acidosis. This rumen dysfunction affects rumen microbes and results in less efficient digestion, thereby decreasing feed intake and exacerbating the energy deficit in the cows. The main way to limit the risk of acidosis is by diet management. Thus, animals must be progressively adapted to grain ingredients during the 3-week period before calving. Furthermore, the diet management strategy will be to privilege low-degradable starch and include enough fiber in the diet to stimulate rumination and salivation. Chemical additives can be used to prevent rumen acidosis. Rumen pH can be controlled by direct addition of chemical buffers at doses of 1-2% of DM intake. Metabolic hydrogen can be mobilized through specific metabolic pathways such as propionogenesis to compete with the synthesis of lactic acid. This can be achieved by adding propionate precursors such as aspartate, malate or fumarate. However, these additives are not economically viable, and will not easily be accepted by consumers. Probiotics, which are mainly supplied as live yeasts, have to be regarded as a solution for preventing subacute acidosis but they are not considered as an efficient cure for acute acidosis. Although the mechanisms of action of probiotics are not fully understood, this paper proposes a plausible model of their mode of action. Recently, additives based on plant extracts have been proposed as a solution for optimizing rumen functions, but further efficacy studies and safety trials are required before these additives can be marketed. The addition of fibrolytic enzymes has been suggested to improve the digestion of the dietary fiber fraction during acidosis. Although the results obtained so far are encouraging, more ruminant-specific research needs to be carried out on these enzymes. Some antibiotics are potential inhibitors of Gram-positive bacteria, which are involved in rumen acidosis, but their use as feed additives has been banned from 01/01/2006 in the EU.