The study with broiler chicks (McCoy et al., 1994) found results similar to those of the nursery pig study. Birds receiving feed of greater uniformity showed significantly increased average daily gains (22 percent). There was also a 6 percent improvement in feed efficiency as the F/G ratio declined from 1.82 to 1.72. The researchers measured mixing by revolutions rather than minutes, with the number of revolutions ranging from five to 80. The significant improvements in performance traits were achieved at 20 revolutions, when the CV for salt had decreased to 12.1 percent, compared to a CV of 40.5 percent at five revolutions. No further improvements in animal performance were noted with 80 revolutions, when the CV declined to 9.7 percent.
In the broiler study, the cost of lower performance with poorly mixed feed continued past the initial four weeks of the study. When all the birds were shifted to a common, uniform finisher diet, those that had received the less uniform feed were unable to compensate weight gain with three more weeks on feed.
The markedly different results between the finishing pig study on the one hand and the nursery pig and broiler chick studies on the other may suggest an interaction between animal age or metabolic body size and nutrient uniformity, Behnke says. A logical hypothesis, he adds, is that younger animals may be more sensitive to nutrient variation than larger or mature ones simply because of the total amount of feed consumed. However, Behnke cautions against assuming this is a reason to discount the importance of nutrient uniformity for more mature animals. The basic premise that each aliquot, or mouthful, of the diet should be balanced for the nutrient requirements of the animal still holds, he says.
Traylor et al. (1994) point out that adequate mixing should be assessed by the CV of the diet rather than mixing time. Mixing is an activity taken on the feed and does not necessarily mean homogeneity or uniformity, which is measured by CV. For instance, the researchers note that feed uniformity with their mixer may have been greater than in a simpler system (such as a grinder-mixer) because substantial blending occurred during the discharge cycle from the mixer.
Meanwhile, surveys of on-farm feed mixing in Kansas found that most producers do a good job of feed mixing, but the surveys also suggest there is room for improvement. Stark et al. (1991) reported that about 42 percent of the samples had a CV of less than 10 percent, 46 percent of samples had between 10 and 20 percent CV, and 12 percent of samples had above 20 percent CV.
Another survey (Herrman and McClure, 1995) found that the CV of feed from participating Kansas pork producers averaged 12 percent but ranged from a low of 4 percent to a high of 34 percent. All the participants did conduct some routine quality tests of feed ingredients. However, the researchers reported that producers seldom sought outside advice on feed manufacturing practices, and only 20 percent reported that they performed assays on finished feed.
Behnke says producers should have each type of finished feed assayed at least once a year for uniformity, and individual ingredients should be assayed more regularly for accuracy. As the mixer ages, producers should take even greater care to retest for feed uniformity and reassess minimum mixing times. Any substantial formula change will also warrant retesting the mixer for uniform feed production. Behnke also adds another caution about accurate measurement and feed uniformity. Mixer capacity is actually determined by volume, although it's commonly described in terms of tonnage. Thus, there is a real risk of overfilling when grain is lightweight--and no amount of mixing will produce uniform feed in an overfilled mixer.
Meanwhile, the 1995 survey of Kansas pork producers found that accurate weighing presented a challenge to producers. None used a scale accurate to 0.1 lb. When the researchers assayed finished feed, they found concentrations for some lower-level ingredients at only 85 percent of the target level. This discrepancy was large enough to justify the cost of more exact scales.
References:
- Behnke, K.C. The importance of nutrient uniformity to animal performance. Kansas State University (Report).
- Herrman, T.J., and G. McClure. 1995. How well is feed made on the farm? Feed Management. 46(8):23.
- McCoy, R.A., et al., 1994. Effect of mixing uniformity on broiler chick performance. Poult. Sci. 73:443.
- Stark, C. R., et al., 1991. On-farm feed uniformity survey. Swine Day Report No. 641. Kansas State University.
- Traylor, S.L., et al., 1994. Mix time affects diet uniformity and growth performance of nursery and finishing pigs. Kansas State University (Report).
