| Summary: | Two studies were carried out to determine if nitrogen excretion could be
reduced through ration formulation, while maintaining milk production in dairy cows.
In each study, 18 lactating Holstein cows were randomly assigned to treatment
sequences in a 3 x 3 Latin Square design, replicated 6 times. Milk, blood, and rumen
fluid samples were taken during the third week of each 28 day experimental period in
the first experiment, and during the second and third week of each 28 day period in
the second experiment. Total collection of urine and feces occurred during the last 5
days of each experimental period.
In the first experiment, total mixed rations were formulated to contain 15.3 (A),
16.4 (B), and 12.3% (C) crude protein. Ration A (control) contained a standard 18%
dairy concentrate, ration B was balanced in terms of protein and carbohydrate
degradation and rates of passage using the Cornell Net Carbohydrate and Protein
System (CNCPS), and ration C was a low protein diet balanced to meet the amino
acid requirements of the cows using the CNCPS model. There was a reduction
(P<0.05) in dry matter intake among cows fed diets A, B, and C (22.8, 21.8, and 20.8
kg d⁻¹, respectively). Milk production and milk protein output were lower (P<0.05) for
the cows fed the low protein diet than for the other two diets, but N efficiency
expressed as milk nitrogen as a % of intake nitrogen tended to be greater (P>0.05) for
animals on the low protein diet. Blood urea nitrogen values were different (P<0.05)
among treatment groups (14.0, 19.1 and 6.8 mg dl⁻¹ for diets A, B, and C,
respectively), but blood non-esterified fatty acid levels were unaffected by dietary
treatment. Fecal nitrogen excretion was lower (P<0.05) for cows fed diet C than for
those fed diet A, but was not different from those fed diet B. Daily urinary nitrogen varied significantly (P<0.05) among the 3 dietary groups (0.154, 0.180 and 0.092 kg
d"1, for diets A, B, and C, respectively). Nitrogen balance was not different (P<0.05)
among treatments.
In the second experiment, total mixed rations were balanced in terms of
degradation, rates of passage, and amino acid requirements using the CNCPS model.
Rumen-protected lysine and methionine were used to balance amino acid
requirements in diets B and C. Crude protein levels in the diets were 18.3, 16.7 and
15.3% CP, for diets A, B, and C, respectively. Diet A resulted in higher (P<0.05) dry
matter intakes than diets B and C. Milk production was lower (P<0.05) for cows fed
diets B and C than for cows fed diet A (34.2, 32.8 and 32.8 kg/d for diets A, B, and C,
respectively) but, milk protein output did not differ (P>0.05) among dietary groups.
Nitrogen efficiency, expressed as milk N as a % of intake nitrogen, improved (P<0.05)
for diet B compared to diet A, and for cows on diet C compared to cows on diets B or
A (25.8, 29.3, and 33.1%, respectively). Blood urea nitrogen values were different
(P<0.05) among groups (15.9, 12.9, and 10.0 mg/dl for diets A, B, and C,
respectively). Treatment differences (P<0.05) were seen in plasma arginine,
aspartate, isoleucine, methionine, threonine and valine concentrations. Plasma
methionine levels were (P<0.05) higher for cows fed diets B and C compared to those
fed diet A, but no differences (P>0.05) were seen in plasma lysine concentrations.
Although there was a significant decrease (P<0.05) in apparent crude protein
digestibility among cows fed diets A, B, and C, respectively, urinary nitrogen excretion
was decreased (P<0.05) dramatically (0.264, 0.195, and 0.162 kg d⁻¹, for diets A, B,
and C, respectively). Fecal nitrogen excretion and nitrogen balance were not affected
(P>0.05) by diet. === Land and Food Systems, Faculty of === Graduate
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