Conflicting facts on milk urea

Would John Roche elaborate on his statement that excessive MU (milk urea) does not impede New Zealand dairy cow reproduction?  I would appreciate it if he would comment on the following facts. For example, could he explain what he thinks caused the decrease in fertility discussed in the University of Arizona study, “Effects of Nutrient Metabolism and Excess Protein Catabolism on Dairy Cow Fertility”, by M.L. Rhoads, T.R. Bilby, R.P. Rhoads and L.H. Baumgard? In summary, they found:

• The metabolism of excess dietary protein results in high plasma urea nitrogen (PUN) 

concentrations;

• High PUN concentrations (and ammonia to a lesser extent) are associated with decreased 

fertility in lactating dairy cows;

• High PUN concentrations probably affect fertility via direct toxic effects on the oocyte 

and embryo, alterations in the uterine environment and decreased progesterone secretion;

• The effects on fertility may be further exacerbated by negative energy balance and heat 

stress.

Fact: excess crude protein influences both blood urea nitrogen (BUN) and MUN. Noelia Silva-del-Rio, of Davis Universtity, California’s Veterinary Medicine and Teaching Research Centre (VMTRC) also says: “Blood urea is freely diffusible to milk, and therefore, MUN reflects the urea concentration in blood”.  This seems to suggest that excessive urea N is carried equally in the blood and in the milk so the blood supplying the cow’s uterus would be carrying excessive urea N.  The question is does that impede the function of the uterus in terms of ova fertilisation?

 Fact: excessive cow PUN influences uterus pH. This is stated in a paper headed “Relationships of Dietary Protein and Fertility” by W. Ronald Butler of Cornell University. Uterine pH normally increases from about 6.8 at oestrus up to 7.1 on d 7 of the oestrous cycle (luteal phase), but this increase failed to occur in both heifers and lactating cows fed excess RDP or RUP (high PUN). 

Fact: New Zealand dairy cows are exposed to excessive BUN and MUN levels. Dairy NZ’s senior scientist Dr Jane Kay, in a media statement on milk urea last December acknowledged high MUN, and therefore PUN, in dairy cows fed on NZ pasture based systems. In New Zealand pasture-based systems, MU levels are naturally much higher than in systems where cows are fed a total mixed ration, it stated.

Fact: Dairy cow conception rates have been dropping despite hormone treatment intervention. In a report by Dairy NZ predecessor Dexcel, Chris Burke and Christine Fowler noted: “Survey data indicates that routinely available conception rates of 65% that were available in the 1970s have eroded to 55% or less with the modern cow”.

Fact:  Urea fertiliser N use increased by 162% between 1996 and 2002 (Parliamentary Commissioner for the Environment, Farming trends, chapter 3.

Fact: Urea fertiliser N increases grass crude protein content. In a South Island Dairy Event paper Kolver and Muller (1998) calculate the energy cost of excreting surplus protein in the form of urea as 0.1kgMS. For a high producing dairy cow more than 25 % protein is not beneficial. Pasture with more than 30 % protein (when pasture is the only feed) can also cause reduced fertility. In these cases, adding a carbohydrate-source to the diet, eg maize- or cereal-silage to the diet can be beneficial. Another option is to not apply high amounts of N-fertiliser (more than 50 kg N) in one hit, but apply smaller quantities distributed over the growing season, they say.

• Steve Clark is an ex-farmer from Wainui, Eastern Bay of Plenty.