[The necessity of a surplus in the nitrogen balance in the rumen of cows having high milk production].
The amount of crude protein entering the small intestine (nXP) and the nitrogen balance in the rumen (RNB) are the two components of the German protein evaluation system. Although there is no scientific evidence, feeding strategies for the practice imply an excess in RNB, particularly in diets for high-yielding cows. This study was undertaken to investigate whether cows benefit from an excess in RNB under conditions of sufficient nXP supply. Three rations were used equal in energy (7.2 MJ net energy for lactation per kilogram dry matter; DM) and nXP (173 g/kg DM) and mainly based on silages from maize, grass, and brewer's grains as well as ground barley and solvent extracted meals from rapeseed and soybeans. Three rations were calculated to allow for an RNB of either 0 or 50 g/day. RNB was adjusted by either a supplementation of urea or a change in the proportions of oilseed meals. The intended levels of energy, nXP and RNB in the diets were confirmed under consideration of separate digestibility measurements with wether sheep and a chemical fractionation of dietary crude protein. The rations were offered as total mixed rations. At least 35 cows between days 21 and 98 of lactation were used per treatment. Cows were milked three times daily and had continuous access to feed and water. Feed intake was measured continuously and for each cow individually by a feeder system through which the rations were offered. The average values for the three treatments ranged between 20.1 and 21.7 kg/day for DM intake, 43.0 and 44.0 kg/day for milk yield, and 1.39 and 1.42 kg/day for milk protein yield. No significant treatment effect was detected in any of the monitored criteria. Cows were in a negative energy budget as indicated by a loss in backfat thickness, similar for all treatments. Under inclusion of literature data it is concluded that under conditions of a sufficient nXP supply, dairy cows do not need an excess in RNB in total mixed rations, even at very high levels of milk protein yield.
Kluth
H
H
Institut für Ernährungswissenschaften, Martin-Luther-Universität Halle-Wittenberg, Germany. kluth@landw.uni-halle.de
Engelhard
T
T
Rodehutscord
M
M
D000821:Animal Feed; D000818:Animals; D002417:Cattle / Q000378:metabolism / Q000502:physiology*; D004044:Dietary Proteins / Q000008:administration & dosage*; D004305:Dose-Response Relationship, Drug; D005260:Female; D007421:Intestine, Small / Q000378:metabolism; D007774:Lactation / Q000378:metabolism*; D008892:Milk / Q000378:metabolism; D008894:Milk Proteins / Q000032:analysis; D009584:Nitrogen / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism*
D004044:Dietary Proteins; D008894:Milk Proteins; D009584:Nitrogen