Feeding Systems with Legumes to Intensify Dairy Farms - cgiar

Defining Milk Urea Nitrogen Concentrations for Optimum
Recommendations of Protein-to-Energy Ratios in
Tropical Forage Diets
Patricia Avila and Carlos Lascano
CIAT, Colombia
When there is an excess of nitrogen relative to energy in the rumen,
ruminal ammonia concentration increases. Unused ruminal ammonia
enters the portal blood through the rumen wall and is transferred to the
liver where it is detoxified by conversion to urea. The liver also produces
urea from deamination of amino acids rising from postruminal digestion and
systemic protein turnover. Urea then circulates in the blood to the kidneys
and is excreted with the urine or it can be diffused from the blood into milk.
When there is a deficiency of dietary proteins, ruminal ammonia
concentrations are relatively low and the proportion of nitrogen recycled
back to the rumen as urea increases. As a result of these metabolic
transactions, blood urea nitrogen (BUN) is highly correlated with ruminal
ammonia and milk urea nitrogen (MUN). Therefore, in healthy ruminants,
MUN concentrations could be a good indicator of the protein-to-energy ratio
in the diet.
The study aimed to determine the level of urea in the milk that could be
used as reference to increase the protein content to supplement diets, with a
high probability that cows will respond by increasing milk production.
Materials and Methods
The study was carried out between 1992 and 1995 at the CIAT-
Quilichao experiment station on contrasting pastures of Andropogon
gayanus and Brachiaria dictyoneura. Two groups of four Brahman x
Holstein cows were submitted to grazing on each pasture, with a 7-day
period to adjust cows to the diet and 7 days of measurements. Milk yields
were recorded during the 7-day measurement period, and milk samples for
urea content were taken on days 1, 4, and 7 of each period. A 4 x 4 Latin
square experiment design was used.
Results
Figure 1 shows the relationship between milk yield increase and MUN
of cows supplemented with Cratylia argentea and sugarcane. Four groups
can be identified, one in each quadrant. In the first group, cows showed an
46