Common contemporary diet formulation methods involve factorial or empirical models which
simulate ruminal fermentation and post ruminal absorption to predict nutrient supply and needs.
However, due to their inability to encompass all animal factors that affect digestion and
absorption, metabolic models inadequately predict microbial protein (MCP) synthesis in the
rumen and passage of nutrients such as protein and amino acids (AA) to the small intestine.
Practical and simple on-farm methods to obtain “real time” values directly from cows are
required to establish normal ranges of MCP flow from the rumen and plasma AA concentrations on commercial dairy farms. Urine purine derivative (PD) output, an index of MCP supply to the
intestine when analyzed in spot urine samples, can accurately predict MCP flow from the rumen
under farm conditions. Blood sampling from the tail vein is easily performed on commercial
dairy farms and concentrations of free AA in these plasma samples, representative of intestinally
absorbed AA, can be used as an index to predict limiting AA. A group of 20 commercial dairy
farms, milking 2677 + 372 cows either 2 or 3 times a day were selected and one of their early
lactation pens holding 255 + 20 cows were used to represent the range of nutritional strategies
and rations fed to California dairy cows. On the day of sampling, one load of early lactation total
mixed ration (TMR) was sampled and 20 pre-selected cows were body condition scored (BCS).
Directly after scoring, the 4 cows with the highest, and the 4 cows with the lowest BCS values
were removed to collect 12 tail vein blood samples from the group with average BCS scores.
Spot urine samples were collected from all voluntarily urinating cows (retaining 6 to 12
samples/pen to match the characteristics of the cows pre-selected for BCS scoring and blood
sampling). Most rations contained alfalfa hay, corn silage, almond hulls, corn dried distillers
grains, corn grain, cottonseed, canola meal (solvent) and a mineral premix. Selected cow groups
produced 45 ± 1.2 kg milk/day at 73 ± 0.5 days in milk with a BCS of 2.6 + 0.04 at 2.8
lactations. There were no correlations between MCP and milk production, but MCP flow from
the rumen was correlated to organic matter and neutral detergent fiber content of the TMR
(P<0.01 and P=0.03 respectively). Plasma AA concentrations were correlated with rumen
undegradable crude protein (CP) and starch content of the TMR, with many inter-correlations
among AA, but no plasma AA concentration was correlated to milk production. Results show
that 8 urine samples and 6 blood samples/group provided accurate representation of the group.
This study documents ranges of MCP flowing from the rumen (1703 ± 54.6 g CP/day), and
plasma AA levels (with low variation within AA), in early lactation multiparity Holstein cows fed a range of contemporary dairy rations with multiple ingredient profile combinations. Since
farms selected were well managed operations with cow groups selected to represent averages in
early lactation, this data can be used as a benchmark for high, low and mean levels of MCP flow,
and plasma AA concentrations, as well as provide real time evaluation of rations to identify
possible rumen microbial growth and/or absorbable AA issues in commercial dairy cows.