Variability in nutrient content or total solids percentage in waste milk can occur if the proportion of transition milk to mastitic milk changes. Nutrient content will decrease as more mastitic milk is added. One possible remedy for this situation is to fortify waste milk with milk replacer powder to increase the nutrient content. However, inaccurate measurement of the supplemental milk replacer may only exacerbate the variability problem. To avoid this, one must be able to make precise measurements of the milk replacer to be added.

On the same issue of consistency, it is important when mixing milk replacer to be accurate in the amount fed per feeding and temperature of the milk replacer. Students in a Dairy Management class at the Pennsylvania State University recently participated in a laboratory assignment that shed some light on the variability that can occur when mixing milk replacer for feeding calves. We used a standard commercial milk replacer and the directions on the bag along with the measuring cup supplied in the bag.

To follow the manufacturer’s instructions correctly one would have to mix 10 ounces of milk replacer powder with 2 quarts of water; water temperature should be 110 to 115°F. These directions are similar to what one may find on the tag of many milk replacers. Assuming the directions are followed precisely, the end result should be a warm, milk-like solution that is 13% solids. Measurement, however, is only as precise as the instrument used. On most dairy farms, a balance, graduated cylinder, and thermometer are not standard equipment for mixing milk replacer. Rather, a plastic cup is used to measure milk replacer powder, water volume is measured in a bottle or pitcher, and the calf feeder’s finger is used to measure water temperature.

During the laboratory sections, students were given the directions above and asked to mix milk replacer for a single calf using the standard farm “equipment.” After mixing, temperature was measured in the final product using a thermometer and total solids were calculated by measuring the total protein percentage with a refractometer and adding 2 (a rule of thumb based on the equation published by Moore et al., 2009). The same cup, provided in the bag of milk replacer powder, was used by all students; however, results were variable (Table 1).

### Table 1. Final temperature and total solids of milk replacer mixed by students

Mean Std Dev Min Max
Temperature (°F) 95.6 9.3 80 115
Total Solids (%) 9.6 2.3 6 14.5

Of the 41 batches of milk replacer mixed, only 17 achieved a final total solids percentage within the goal range of 10 to 15%. Only two calves would have received a final solution with total solids percentage of 13%. When students measured the final temperature of the milk replacer, only 2 of 41 achieved a final temperature between 110 and 115°F. Many of the students in the class were from dairy farms and had previous experience mixing milk replacer. This in-class experiment simply shows the variation that can occur when imprecise equipment is used.

In contrast to the variation seen in mixing milk replacer, recent data from a Pennsylvania State University field trial showed relatively little fluctuation in raw milk total solids. Samples of waste milk were collected from 9 Pennsylvania dairy farms twice a day for 15 days, and total solids were measured using the same rule of thumb mentioned above. When data from all 9 farms were combined, total solids ranged from 8.3 to 13.9%. However, on any single farm, variation was much lower (Table 2).

## Table 2. Variation in waste milk on 9 dairy farms in Pennsylvania

Farm Mean St Dev Min Max
1 10.7 0.3 10.1 11.2
2 10.5 0.9 9.4 13.9
3 10.1 0.3 9.3 10.8
4 10.5 0.8 8.6 11.7
5 10.3 0.5 8.3 11.0
6 10.1 0.5 9.0 10.9
7 10.2 0.2 9.8 10.7
8 10.8 0.4 10.2 11.8
9 10.3 0.2 9.7 10.6

In these two examples, variation in total solids resulting from imprecise measurement of milk replacer powder was approximately 2.5 to 11.5 times greater than the variation in the waste milk that we try to correct by fortification. So, how can we decrease this variation and ensure that calves receive a consistent product at each feeding? We need to improve the precision with which we measure our ingredients. Some recommended equipment that is both economical and easy to maintain are a simple balance and a bowl large enough to hold the maximum amount of powder to be mixed, a measuring cup that holds just what is needed to be mixed, a cooking thermometer to ensure a consistent water temperature, and a pitcher or bottle with graduated markings for measurement. A hand-held refractometer for measuring total protein or serum protein may also be used to ensure that total solids percentage of the final product is within the desired range.

### Reference

• Moore, D. A., J. Taylor, M. L. Hartman, and W. M. Sischo. 2009. Quality assessments of waste milk at a calf ranch. J. Dairy Sci. 92:3503-3509.