Effects of rumen-protected methionine in a low protein ration on metabolictraits and performance of early lactating cows as opposed to rations with elevated crude protein content

A 5-week experiment with 24 multiparous early lactating Brown Swiss cows wa s conducted to investigate the effects of supplementary rumen-protected met hionine in conjunction with dietary protein reduction on metabolism and per formance after 1 week of control measurement. Three rations containing 175, 150 and 125 g of crude protein/kg feed dry matter were supplemented with m ethionine. The fourth ration, also only containing 125 g of crude protein/k g dry matter, remained unsupplemented. The four treatment groups had a simi lar metabolic supply of other essential amino acids, protein and energy, as calculated by various approaches. The two low protein rations were, howeve r, slightly deficient in ruminally degraded protein. Treatment effects rema ined low on feed intake, forage meal pattern, milk yield and fat as well as lactose content. In contrast, the content and yield of milk protein signif icantly declined only in the unsupplemented low protein ration relative to the initial value. Compared with this ration, the decline in milk protein y ield was clearly delayed in the supplemented low protein ration. Blood plas ma methionine tended to be reduced without supplementation and to be increa sed with additional methionine. Supplementation of methionine reduced other plasma amino acids. Plasma insulin, glucose, lactate, ketone bodies and as partate amino transferase activity indicated a certain liver stress and a s omewhat elevated energy requirement with high and particularly with ION pro tein content (when unsupplemented). Methionine improved metabolic protein u tilization, followed by the lowest plasma, urine and milk urea levels in th e supplemented low protein diet. In conclusion, no major adverse effects we re assessed under the conditions tested. Supplementation of methionine may nevertheless be useful in rations with particularly low protein content fed to early lactating cows in order co prevent negative long-term effects whi ch were only visible here as trends.