Effect of sample preparation method and drying method on the concentration of energy in urine and the concentration of metabolizable energy in diets fed to pigs

To obtain accurate values for metabolizable energy (ME) in diets and ingredients, an accurate estimation of gross energy (GE) in urine is required. Urine samples are often prepared following the freeze-dried cotton-plastic bag method and GE is determined by the ignition of the sample in a bomb calorimeter. However, the concentration of GE in urine can also be determined by dripping urine on a cellulose pellet, which can then be ignited in a bomb calorimeter. Alternatively, energy in urine can be calculated from the concentration of N in urine assuming that all energy in urine originates from N. The latter 2 methods may be less expensive and less time consuming than using the freeze-dried cotton-plastic bag method. Likewise, oven drying instead of freeze drying of samples may reduce time and cost of the drying procedure. However, to our knowledge, no data comparing values for GE in urine and ME in diets among different sample preparation and drying methods are available. Therefore, the objective of this study was to test the hypothesis that there are no differences in GE of urine or ME values of diets among sample preparation methods (freeze-dried cotton-plastic bag, undried cellulose pellet, or N value method). The second hypothesis was that drying method (freeze drying or oven drying) of cotton-plastic bag urine samples from pigs do not influence analyzed the GE in urine or calculated ME in diets.

 

Experimental Procedures

Eight diets were used. The concentration of digestible energy (DE) in the diets was calculated using data for the concentration of GE in diets and in feces from pigs fed these diets. Urine samples were frozen after collection and used to determine effects of different sample preparation and drying methods on the concentration of GE in urine and ME in diets. The main effects of diet and sample preparation or drying method were analyzed because no interactions between main effects were observed.

Freeze-dried cotton-plastic bag vs. undried cellulose pellet. Urine samples from pigs fed diets containing corn, corn and fermented soybean meal (FSBM), or corn and soybean meal (SBM) were collected. For each sample, 4 sub-samples were prepared using the conventional freeze-dried cotton-plastic bag method and 4 additional sub-samples were prepared using the undried cellulose pellet. Samples were analyzed for GE using a bomb calorimeter and the concentration of ME in the diets was calculated using the direct approach.

Freeze-dried cotton-plastic bag vs. N value method. Urine samples from pigs fed 3 different diets were collected. The diets contained corn and SBM, corn, SBM, and full fat rice bran (FFRB), or corn, SBM, and defatted rice bran (DFRB). Four sub-samples were prepared using the conventional freeze-dried cotton-plastic bag method, whereas the concentration of N in urine was analyzed in duplicate. The N value was used to calculate GE in urine.

Freeze-dried vs. oven-dried cotton-plastic bag. Urine samples were collected from pigs fed 2 diets. One diet contained corn and SBM and the second diet contained corn, SBM, distillers dried grains with solubles (DDGS), and wheat middlings (WM). Eight sub-samples were prepared using the conventional freeze-dried cotton-plastic bag method. Four sub-samples were lyophilized in a freeze dryer for 8 days, whereas the other 4 sub-samples were oven dried at 60 oC for 5 days in a forced-air oven.

 

Results

Freeze-dried cotton-plastic bag vs. undried cellulose pellet. The concentration of GE in urine obtained using undried cellulose pellets was greater (P < 0.05) than the value obtained using freeze-dried cotton-plastic bags (Table 1). As a consequence, the calculated ME in diets was lower (P < 0.05) if the undried cellulose pellet method was used instead of the freeze-dried cotton-plastic bag method.

Freeze-dried cotton-plastic bag vs. N value method. The concentration of ME in the corn-SBM diet was lower (P < 0.05) than in the corn-SBM-FFRB diet, but greater (P < 0.05) than in the corn-SBM-DFRB diet (Table 2). The value for GE in urine obtained by the N value method tended (P < 0.10) to be lower than the value obtained by the conventional freeze-dried cotton-plastic bag method. Therefore, the N value method resulted in increased (P < 0.05) concentration of ME in diets compared with the conventional freeze-dried cotton-plastic bag method.

Freeze-dried vs. oven-dried cotton-plastic bag. There was no effect of drying method on the concentration of GE in urine or the calculated concentration of ME in diets (Table 3).

 

Key points

  • The cellulose pellet method provided values for the GE in urine that were greater than if the conventional freeze-dried cotton-plastic bag method was used, which resulted in an underestimation of ME in diets.
  • Using the concentration of N in urine results in estimated values for the concentration of GE in urine that are lower than values obtained from freeze-dried cotton-plastic bags. Therefore, the calculated ME in diets was overestimated.
  • The difference in the ME of diets between the freeze-dried cotton-plastic bag method and the undried cellulose pellet or the N value methods is less than 100 kcal/kg.
  • There was no effect of drying method on the concentration of GE in urine or ME in diets fed to pigs. Therefore, it appears that oven drying of urine dripped on a cotton bag inside a plastic bag can be used and freeze drying is not necessary. 

 

Table 1. Effect of sample preparation method (freeze-dried cotton-plastic bag or undried cellulose pellet) on concentrations of gross energy (GE) in urine and metabolizable energy (ME) in diets containing corn, corn and fermented soybean meal (FSBM), or corn and soybean meal (SBM) fed to pigs

1Data are least square means of 16 observations.

2Data are least square means of 24 observations.

 

Table 2. Effect of sample preparation method (freeze-dried cotton-plastic bag or nitrogen value) on concentrations of gross energy (GE) in urine and metabolizable energy (ME) in diets containing corn and soybean meal (SBM), corn, SBM and full fat rice bran (FFRB), and corn, SBM, and defatted rice bran (DFRB) fed to pigs

1Data are least square means of 16 observations.

2Data are least square means of 24 observations.

 

Table 3. Effect of drying method on the concentration of gross energy (GE) in urine and the concentration of metabolizable energy (ME) in diets containing corn and soybean meal (SBM) or corn, SBM, distillers grain with solubles (DDGS), and wheat middlings (WM) fed to pigs

1Data are least square means of 14 and 12 observations for the Corn-SBM and Corn-SBM-DDGS-WM diets, respectively.

2Data are least square means of 13 observations.

 

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