Graduate Thesis Or Dissertation
 

The effects of various levels of dietary protein on high producing dairy cows in early lactation

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  • 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 period one. 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.
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