Some animals, such as cows, normally make only one offspring from each mating. If a cow showed a dominant phenotype, why would a typical testcross be a complex way to verify the genotype of that animal?
In a testcross, the dominant phenotype would appear in all of the offspring if the cow were homozygous dominant but in only about 50% of the offspring if the cow were heterozygous. With only one individual per F1 generation, distinguishing among these two possibilities would take a long time, unless a calf with the recessive phenotype was born.