Ian Sawyer, Ruminant Technical Services, Feedworks Australia and adjunct fellow at the School of Agricultural and Veterinary Science at University of Melbourne.
Excess rumen ammonia is converted to urea and then secreted in the urine (and the milk for lactating cows) or it can be re-utilised via recycling through saliva. When protein levels are low in the diet, nitrogen cycling activity will be increased compared to when protein levels in the diet are in excess of animal requirements. This is a survival mechanism for conserving nitrogen when protein levels in the diet are low.
When protein levels in the diet are high, it costs an animal to get rid of excess nitrogen. Predictions of the cost of this to animal production can be made by many diet balancing models. A good rule of thumb is 7.3 calories per gram of excess protein (i.e. over 16% CP in dairy diets). The process to eliminate excess nitrogen also costs amino acids.
Ammonia surges can also have a negative impact on dry matter intake as it can have hypophagic, reduction in feed intake and eating behaviour, effects. There is also work showing negative impacts on fertility with ammonia surges, with the main impact being on the blastocyst cleaving in early pregnancy. In practice, this can look like more returns following mating.
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