Experimental Trials Assessed by Two Different Protein Evaluation Systems

Abstract

Twentythree dairy cows were fed rations with different proportions of energy and digestible crude protein (DCP). When the ration was balanced for energy and DCP according to Swedish standard the cows’ milk urea concentration was 4.66-4.92 mmol/1 (95% CI of mean). With increasing intakes of DCP, fed together with standard levels of energy, the mean milk urea concentration increased in proportion to the surplus of DCP. In contrast, the concentration of urea decreased when the cows were overfed with energy at the same time as they were underfed with protein.

When the rations were recalculated in accordance with the AAT/PBV system for dietary protein evaluation the 95% CI for the mean milk urea concentration of the cows receiving a balanced ration was 3.76-4.56 mmol/1. The concentration of urea was dependent primarily on the PBV. When the 2 protein evaluation systems were compared there was a strong correlation between PBV and DCP. Ammonia was the only constituent of the rumen whose concentration was strongly correlated with the milk urea concentration.

Taken together with earlier data the present results suggest that a milk urea concentration between 4.0 and 5.5 mmol/1 should be regarded as normal at least when cows are fed conventional feedstuffs.

Sammanfattning

Tjugotre mjolkkor gavs foderstater med olika mangder energi och smaltbart råprotein (DCP). Når energi- och DCP-tillforseln motsvarade behovet enligt svensk norm var mjolkens ureahalt i medeltal 4,66-4,92 mmol/1 (95% konfidensintervall for medeltalet). Nar mångden DCP okades utover behovet samtidigt som energitillforseln motsvarade behovet, steg mjolkureavardet ungefår ι proportion till DCP-overutfodringen. Nar korna overutfodrades med energi och samtidigt underutfodrades med DCP sjonk mjolkureakoncentrationen.

Nar foderstaterna retrospektivt områknades till AAT/PBV-systemet for proteinvårdering blev mjolkureahalten i medeltal 3,76-4,56 mmol/1 (95% CI) nar foderstaten var balanserad enligt norm. Mjolkureakoncentrationen var fråmst beroende på foderstatens PBV-innehåll. Nar de två proteinvårde-ringssystemen (DCP och AAT/PBV) jämfördes forelåg en stark korrelation mellan PBV och DCP. Av ett flertal våmparametrar befanns endast ammo-niakkoncentrationen vara signifikant korrelerad till mjolkureahalten.

Resultaten från detta forsok och från tidigare un-dersokningar indikerar att en mjolkureakoncentra-tion på 4,0-5,5 mmol/1 kan betraktas som normal hos mjolkkor som bjuds en konventionell stallfoderstat.