New approaches, development, and improvement of methodologies for the assessment of B-vitamin requirements in dairy cows

Studies on B-vitamin requirements of cattle, conducted more than 60 years ago, concluded that mature ruminants do not require B-vitamin supplements because the amounts of vitamins provided by the diet and synthesized by the ruminal microflora were sufficient to prevent emergence of deficiency symptoms. As a result, the impact of subclinical deficiency on maintenance of normal and efficient metabolism has been disregarded and very little research effort has been devoted at defining dairy cow requirements for B vitamins. However, emergence of deficiency symptoms is the last stage of the deficiency; deficiency appears as soon as the supply is inferior to the needs, leading to a loss of metabolic efficiency. As B vitamins play critical roles in carbohydrate, protein, and lipid metabolism, it is likely that the demand for these cofactors increases with milk yield. Reports over the last two decades of beneficial effects of B-vitamin supplementation, such as thiamin, niacin, biotin, folic acid, and vitamin B 12, suggested that, under some conditions, the need for B vitamins exceeds the supply from the diet and the synthesis by rumen microbes, leading to sub-optimal milk production and metabolic efficiency. However, responses to B-vitamin supplementation are highly variable. The major challenge faced by studies on B-vitamin requirements of dairy cows is the very limited knowledge on dietary factors driving the fate of B vitamins in rumen. This knowledge is essential to identify the conditions under which the dairy cow could benefit from B-vitamin supplements. The present review aims to describe the present state of knowledge on B-vitamin requirements of dairy cows as well as some of the major problems that need to be overcome to progress in this research field.