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

ABSTRACT 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 B12, 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.

摘要 六十余年前开展的反刍动物B族维生素需求相关研究得出结论：成熟反刍动物无需额外补充B族维生素，因为日粮提供的维生素以及瘤胃微生物群落合成的维生素量足以避免缺乏症症状的出现。因此，亚临床缺乏对维持正常且高效代谢的影响被忽视，而针对奶牛B族维生素需求的研究投入也极少。然而，缺乏症症状的出现是维生素缺乏的终末阶段；当供给量低于需求量时，即会出现维生素缺乏，进而导致代谢效率下降。由于B族维生素在碳水化合物、蛋白质及脂质代谢中发挥关键作用，这些辅酶的需求很可能随产奶量增加而上升。近二十年来，关于补充B族维生素（如硫胺素（thiamin）、烟酸（niacin）、生物素（biotin）、叶酸（folic acid）及维生素B12（vitamin B12））有益效果的研究报告表明，在某些条件下，奶牛对B族维生素的需求超过了日粮供给及瘤胃微生物合成的量，进而导致产奶性能与代谢效率欠佳。不过，针对B族维生素补充的响应性存在极大个体差异。当前奶牛B族维生素需求相关研究面临的主要挑战，是对调控瘤胃内B族维生素代谢去向的日粮因素认知极为有限。该认知对于明确奶牛可从B族维生素补充中获益的条件至关重要。本综述旨在阐述当前奶牛B族维生素需求的研究现状，以及该研究领域取得进展所需攻克的若干核心难题。