Genetic and Environmental Changes Possible in Milk Composition

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Abstract
The possibility of changing milk con- stituent percentages in lactation records through environment is small. Fat per- centage can be reduced by feeding high- concentrate low-roughage rations, low- fiber feeds, pellets, etc. Changes in milk composition are pos- sible through breeding, as heritabilities for yields are around .25, percentages .50 to .60, and ratios .60, but gains in protein and solids-not-fat (SNF) per- centages would be less than fat gains because of their smaller variation. Genetic correlations among yields are highly positive, .9; between percentages, .3 to .9%; and between yields and per- centages generally negative, --.2 to --.4. Selection for protein or SNF percent- age alone would be effective but would result in a correlated loss in milk (75 to 100 kg per generation) with small gains in protein or SNF yields. Selection for milk would generally result in a corre- lated increase in protein yield, which is about 90% as effective as direct selection ~or protein yield but would decrease pro- tein percent slightly. However, selection for protein yield gives about 70% as great an increase in milk field as selec- tion for milk alone and increases pro- tein percent about 20~ as much as se- lection for protein only. Selection indices have been constructed so as to main- tain the percentages while improving milk yield. In practice, Holstein dairy- men in this country and in Holland have increased fat percent and milk yield.