| Summary: | A field study was conducted to assess phosphorus (P) and potassium (K)
concentrations of both lactating and dry cow diets on Oregon's dairies. Thirty-seven
dairy farms, located in western Oregon, were grouped according to geographic region,
valley (V) or coast (C), and herd size, small (S) or large (L). Farms were visited on
three separate occasions. During each visit, lactating and dry cow diets were recorded
and corresponding feed ingredients were collected and analyzed for P and K. For each
diet recorded, fecal and urine samples were collected and analyzed for P and K.
When available, milk production data was obtained. During the initial visit, a survey
was issued to producers to assess P and K knowledge as well as gather herd data and
management information. Survey responses received indicated that two-thirds of
participants were knowledgeable about P and its affects within the environment.
Knowledge of K appeared to be less than that of P. Average P concentration of the
lactating cow diet was 0.40% (DM basis) and did not differ between region (P=0.12)
or herd size (P=0.76). Fecal P excretion did not differ between region (P=0.08) or
herd size (P=0.27), however, a trend for larger fecal excretion in V farms contributed
to the lower calculated apparent P digestibility for V than C. Potassium in lactating
cow diet was greater (P=0.01) for C than V, however, but no difference between herd
size (P=0.10) was determined. Overfeeding of K also occurred in dry cow diet with
no difference between region (P=0.40) and herd size (P=0.72). Combining the
fecal and urinary fractions, an individual lactating cow consuming 0.40% P (89.8 g)
and 1.71% K (384.3 g) per day will excrete 24.0 kg of manure P and 76.3 kg of manure
K annually. The magnitude of plant available P and K produced; requires Oregon
producers to acquire additional land, an additional two-tenths ha/cow, to be to apply P
and K at agronomic rates. === Graduation date: 2004
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