Mixing uniformity, as estimated by coefficient of variation (CV) of a specific nutrient or ingredient (usually chlorine from salt), is used to measure degree of dispersion of ingredients throughout a batch of feed. It is often recommended that a coefficient of variation less than 10 percent indicates appropriate mixing, but recent data suggest that up to 20 percent CV may be adequate for most practical diets. In general, mixing uniformity increases with mixing time.
Appropriate mixing time depends on mixer type and ingredient selection. Vertical mixers require considerably more time than horizontal mixers to achieve similar results. It is suggested that mixing times be at least 15 minutes for a vertical mixer, 7 minutes for a horizontal paddle mixer, and 4 minutes for a horizontal ribbon mixer. Also, a batch of feed composed of ingredients with uniform particle size requires less time to mix than a batch of ingredients with greater variation in particle size. Best results are achieved when the mixer is loaded first with a portion of the major ingredients, then with minor ingredients and additives, and finally with the remaining portion of major ingredients. Liquids should be always added only after thoroughly mixing all dry ingredients.
The effects of feed under-mixing on weaned pig performance have been investigated with a complex starter diet mixed for 0, 0.5, 2 and 4 minutes in a horizontal ribbon mixer (Table 1). As expected, pigs offered unmixed diets (ingredients were loaded in the mixer, and without mixing they were discharged in bags) had decreased growth performance compared to pigs offered mixed diets. Yet, performance was not greatly improved when diets were mixed for more than 0.5 minutes, although best performance was achieved with 4 minutes mixing time. In this study, coefficient of variability decreased from 107 to 12 percent as mixing time increased from 0 to 4 minutes, respectively. Because no effects of under-mixing were detected in finishing pigs, it is suggested that mixing uniformity is of greater importance in young pigs, which take rather smaller meals especially during the first few weeks post-weaning.
In broilers, mixing uniformity appears to be most crucial in the first few days post-hatch when feed intake is but grams per day. Two experiments conducted to evaluate mixing uniformity in broilers indicated that overall growth performance was not affected by feed that was adequately- or overly-mixed, but both growth and feed efficiency suffered by poorly mixed feed. In these trials, up to 20 percent CV was considered sufficient. Interestingly, mortality was unaffected by all treatments.