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Optimizing phosphorus utilization by dairy cows
A survey was conducted on 10 dairy farms in Manitoba to quantify factors that affect the utilization of phosphorus (P) by lactating cows. Farms were visited once to collect samples and data, including days in milk (DIM), milk yields (MY), parities (PAR), body condition scores (BCS), and body weights from 30 Holstein cows in different stages of lactation. Feeds were analyzed for crude protein (CP), fat (FAT), ash (ASH), NDF, starch (STARCH), P (DIETP) and acid insoluble ash (AIA). Feces were collected from the rectum and analyzed for P (FECALP) and AIA. Composite milk samples were analyzed for fat (MFAT), protein (MPROT), and P (MILKP). The apparent digestibilities of P (ADCP) of cows were determined using AIA as a marker. Dry matter intake was estimated using the equation of NRC(2001). The P utilization efficiencies (PEFF) of cows were determined as the ratio between the output of P in milk and the P intake. Descriptive statistics (Table 1) show the considerable variation of parameters among cows. Examination of dietary and fecal P concentrations and measures of P utilization suggested that approximately half of the cows received more P than required. Linear regression models were developed between dependent variables (FECALP, PEFF, ADCP) and independent diet and cow variables. Independent variables with a significance level greater than 0.25 were stepwise removed.
Regression models were: ADCP = -152.9+1.00*PAR-2.06*MFAT+2.22*CP+3.63*FAT+1.18*ASH+2.52*NDF+114.06*DIETP (R2=0.39)
PEFF = 45.91-0.45*PAR-0.04*DIM+0.34*MY–2.64*MFAT+313.80*PMILK-82.37*DIETP (R2=0.79)
FECALP = 1.21+0.0005*DIM+0.03*CP-0.10*FAT+0.06*ASH-0.01*STARCH-0.04*NDF (R2 =0.25)
Correlations among independent variables, including PDIET and other dietary variables, complicated assessing relationships between variables. The models show the complexity of factors that determine the efficiency of P for milk production, and generally suggest that this efficiency decreases in later stages of lactation when and milk yield decreases and diets are formulated accordingly.
Table 1. Descriptive statistics |
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Average |
SD |
Min. |
25’th perc. |
50’th perc. |
75’th perc. |
Max. |
PAR |
2.37 |
1.41 |
1 |
1 |
2 |
3 |
8 |
DIM |
170 |
97 |
15 |
74 |
166 |
259 |
464 |
Milk yield, kg/d |
35.6 |
9.0 |
9.5 |
28.7 |
34.4 |
41.3 |
65.1 |
Milk P, % |
0.09 |
0.07 |
0.07 |
0.09 |
0.09 |
0.10 |
0.11 |
Milk P eff., % |
34.6 |
8.3 |
16.1 |
28.3 |
33.3 |
39.5 |
61.4 |
CP, %DM |
16.9 |
0.9 |
15.1 |
16.4 |
16.8 |
17.6 |
18.3 |
NDF, %DM |
34.1 |
3.6 |
29.7 |
30.7 |
33.8 |
35.8 |
41.0 |
P, %DM |
0.41 |
0.04 |
0.34 |
0.37 |
0.41 |
0.45 |
0.47 |
Feces P, %DM |
0.70 |
0.19 |
0.30 |
0.55 |
0.69 |
0.83 |
1.28 |
ADC P, % |
35.1 |
13.2 |
15.9 |
24.0 |
31.7 |
48.1 |
59.7 |
Keywords: phosphorus, effeciency, dairy