- This project was designed to test the biological effects of three
levels of dietary protein (12.7, 16.3 and 19.3% CP on a dry matter
basis) on high producing Holstein-Friesian dairy cows. Forty five
second lactation or older cows that showed the potential to produce
over 31.75 kg (70 lbs) per day were selected. Cows were randomly assigned
to treatments on day four postpartum and remained on the trial
for 90 days. A complete isocaloric ration was fed, consisting of corn
or grass silage, alfalfa hay and a grain concentrate.
Four percent fat-corrected milk production did not differ significantly
between groups averaging 29.4, 31.3 and 32.0 kg/cow/day for the
12.7, 16.3 and 19.3% CP rations, respectively. Actual milk production
was significantly higher in the 16.3% CP group than the 12.7% CP group.
Group averages were 30.94, 35.52 and 34.79 kg/cow/day for the 12.7,
16.3 and 19.3% CP rations, respectively.
Milk fat concentration averaged 3.66, 3.21 and 3.46% for the 12.7,
16.3 and 19.3% CP rations, respectively. The average for 16.3% CP
ration cows was significantly different from the averages of the 12.7
and 19.3% CP ration cows. Milk fat concentration for all cows was 3.7%
for period one (1-30 days), 3.21% for period two (31-60 days) and 3.46%
for period three (61-90 days). Milk fat % was greater (P < .01) for
Milk protein was not significantly affected by level of dietary
protein. Significant differences were found between period one relative
to periods two and three. The period averages were 3.76, 3.36 and
3.46 for period one, two and three, respectively.
Persistancy of lactation was significantly different between periods,
averaging 106.5% in the first period, 100.2% in the'second period
and 98.4% in the third period. Persistancy was not affected by treatment.
Feed intake differed significantly due to treatment averaging 17.7,
18.18 and 18.67 kg/cow/day for the 12.7, 16.3 and 19.3% CP rations, respectively.
There were no significant differences between periods.
Bodyweight changes were not significant between the three treatments,
although they were (P < .01) between periods. During the first
period all cows averaged a 24 kg weight loss; however, cows gained 3.8
kg in the second period and 13.5 kg in the third period.
Rumen fluid ammonia increased with each level of protein (4.29,
7.66 and 19.3 mg% for the 12.7, 16.3 and 19.3% CP rations, respectively),
but was only significant between the 19.3% CP ration relative to
the 16.3 and 12.7% CP rations. Rumen fluid pH did not differ between treatment means.
Blood samples were analyzed for several factors. Plasma urea
nitrogen (PUN) measured at six weeks postpartum, averaged 7.89, 10.27
and 18.25 mg% for the 12.7, 16.3 and 19.3% CP rations, respectively.
Plasma albumin showed the same trend, being significantly higher in
cows on the 19.3% CP ration. Average values were 3.45 mg% for the
12.7% CP level, 3.47 mg% for the 16.3% CP level and 3.85 mg% for the
19.3% CP ration. Blood ammonia levels, determined in 30 cows at various
stages of lactation, were not significantly different.
An economic analysis of the profitability of feeding the various
levels was completed using actual milk and milk fat yields and actual
feed consumption per cow over the 90 day period. Using August, 1977
commercial milk and ration constituent prices, returns above feed costs
per cow for the 90 day periods were $459.58 for the cow fed the 12.7%
CP ration, $477.67 for cows fed the ration containing 16.3% CP and
$456.88 for cows fed the 19.3% CP ration.