SUMMARY

G. L. Cromwell, J. H. Brendemuhl, L. I. Chiba, T. R. Cline, T. D. Crenshaw, C. R. Dove, R. A. Easter, R. C. Ewan, K. C. Ferrell, C. R. Hamilton, G. M. Hill, J. D. Hitchcock, D. A. Knabe, E. T. Kornegay, A. J. Lewis, G. W. Libal, M. D. Lindemann, D. C. Mahan, C. V. Maxwell, J. C. McConnell, J. L. Nelssen, J. E. Pettigrew, L. L. Southern, T. L. Veum, and J. T. Yen. North Central Region Committee on Swine Nutrition (NCR-42) and Southern Regional Committee on Nutritional Systems for Swine to Increase Reproductive Efficiency (S-288).

An experiment involving 25 experiment stations in the North Central and Southern regions (NCR-42 and S-288, respectively) was conducted to assess the degree of uniformity of diet mixing among stations and to assess the variability among station laboratories in chemical analysis of mixed diets. A fortified corn-soybean meal diet was mixed at each station using a common diet formula (except for vitamin and trace-mineral additions). The diet was calculated to contain 14% crude protein (CP), 0.65% Ca, 0.50% P, and 125 ppm Zn (based on 100 ppm added Zn). After mixing, samples were collected from the initial 5% of feed discharged from the mixer, after 25, 50, and 75% was discharged, and from the final 5% of discharged feed. The five samples were sent to the University of Kentucky, finely ground, and divided into subsamples. Each set of five subsamples from each station was distributed to three randomly selected stations for analysis of CP, Ca, P, and Zn (i.e., each station analyzed five diet subsamples from three other stations). In addition, two commercial and two station laboratories analyzed composites of the five subsamples from each of the 25 mixed diets.

Based on the laboratories that analyzed all diets, means were 13.5, 0.65, and 0.52%, and 115 ppm for CP, Ca, P, and Zn, respectively. Ranges of 11.8 to 14.6% CP, 0.52 to 0.85% Ca, 0.47 to 0.58% P, and 71 to 182 ppm of Zn were found among the 25 diet mixes. The coefficients of variation among the 25 diet samples for CP, Ca, P, and Zn were 4.3, 9.3, 4.1, and 17.4%, and among the 25 laboratories were 3.6, 12.5, 10.7, and 11.1%, respectively. Overall analyses of the five subsamples were, respectively, CP: 13.4, 13.6, 13.4, 13.5, and 13.4% (P < 0.06); Ca: 0.66, 0.67, 0.67, 0.66, and 0.67%; P: 0.50, 0.51, 0.51, 0.50, and 0.50%; and Zn: 115, 116, 112, 113, and 120 ppm (P < 0.001). Diets were not uniformly mixed at all stations (station x sample No. was P < 0.08 for Ca and P < 0.01 for CP, P, and Zn). Among stations, the range of the five samples, expressed as a percentage of the mean and averaged for CP, Ca, P, and Zn, varied from ± 1.1% (i.e., 98.9 to 101.0%) to ± 12.9% (84.6 to 110.4%), with an overall average of ± 5.2%. Neither type nor volume of mixers was related to mixing uniformity. The results suggest that uniformity of diet mixes varies among experiment stations, that some stations miss their targeted levels of nutrients (especially Zn), and that the variability among experiment station laboratories in analysis of dietary Ca, P, and Zn in mixed diets is quite large.

This study indicates that the variation among chemical analyses of diets by experiment station laboratories is quite large, especially with respect to zinc analysis. The study also provides evidence that researchers are not always able to achieve a targeted level of a nutrient, such as zinc, that is supplied in the form of a premix. Some stations apparently mix experimental diets quite uniformly while other stations do not. Mixer type or capacity does not seem to affect the efficiency of which diets are uniformly blended. The results raise the possibility that, in some experiments, variation in animal performance across dietary treatments could be due to mixing error rather than animal variation. Care must be taken in nutrition experiments to minimize mixing errors in order to avoid drawing erroneous conclusions regarding dietary treatment effects.

Key Words: Chemical Analysis, Feed Mixing

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