In dairy cows, herbage-based diets often lead to increased nitrogen excretion. Tanniferous sainfoin and extract of acacia can reduce nitrogen excretion from urine and thus ammonia volatilization from slurry.
Ammonia emissions are undesirable in several respects: they represent a nitrogen loss during animal housing, manure storage and soil spreading causing an economic loss for farmers. At the same time, the airborne ammonia (NH3) is redeposited elsewhere in wet or dry form contributing to overfertilization and acidification in semi-natural, sensitive ecosystems such as woodland, extensive meadows, wetland and moorland due to excess nitrogen (N) inputs.
The herbage-based diets fed to dairy cows are often rich in rumen-degradable protein, which results in excessive NH3 production in the rumen. Excess NH3 in the rumen is absorbed and transported in the bloodstream to the liver, where it is metabolized into urea. The urea is then mainly excreted as urinary urea nitrogen (UUN), which is easily released into the environment as NH3.
Tannins inhibit protein degradation in the rumen and thus N excretion
One solution may be to supplement the diet with condensed tannins. These bind to the dietary crude protein and reduce its ruminal degradation rate, thus shifting the N excretion pathway from urine to faeces and reducing the NH3 volatilization potential of the excreta.
Sainfoin and acacia supplementation to reduce NH3
Among the sources of tannin, extract of acacia bark (Acacia mearnsii) and the forage legume sainfoin (Onobrychis viciifolia) were investigated to determine their effects on N excretion in dairy cows and NH3 volatilizationfrom slurry.
Combination of acacia extract and sainfoin increases the effect
Acacia extract and sainfoin in the diet were shown to reduce urinary N excretion and NH3 volatilization from the slurry of dairy cows. Furthermore, the effect of the two tannin sources was found to be additive.
Effect on digestibility and milk yield
The sainfoin silage diet reduced the digestibility of organic matter compared with the ryegrass and clover silage diet. With the addition of acacia extract, the feed intake and digestibility of organic matter was reduced across all silage types under investigation.
Acacia extract reduced the energy-corrected milk yield, while sainfoin silage reduced the milk protein content.
- The tanniferous forage legume sainfoin and a tanniferous extract from the bark of acacia were compared with ryegrass and clover silage to determine their effect on nitrogen partitioning in dairy cows.
- Sainfoin and acacia extract reduced N losses from urine. The combined effects of sainfoin and acacia extract was widely additive in this respect.
- The use of acacia extract as a feed additive reduced ammonia volatilization from slurry by up to 37%.
- Sainfoin silage reduced the digestibility while acacia extract reduced the milk yield.