More milk from every cow
314 words over 29 numbered lines, then 6 parts worth 13 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice unless a source is named.
More milk from every cow
A modern dairy cow can produce more than thirty litres of milk a day,
far more than a calf would ever need. This is the result of many
generations of selective breeding. For thousands of years, farmers
have chosen which animals to breed from, keeping the ones with the
characteristics they wanted.
Milk yield is not controlled by a single gene. It is affected by many
genes, and also by the environment, including the food, the housing
and the health of the cow. Because of this, milk yield shows
continuous variation: there is a range of values rather than a few
distinct groups.
Today a dairy farmer can choose semen from a bull whose daughters on
other farms produced large amounts of milk. A vet uses it to
inseminate the cows with the highest yields in the herd. The female
calves are kept and, when they are grown, the best of them are bred in
the same way. This process is repeated generation after generation.
Figure 1 shows how the mean yield of the cows in one country has
changed.
Over the same period, farmers have also improved the diet of their
cows. They feed them mixtures of grass, maize and protein-rich soya,
and they keep them in sheds in winter. Better food and care allow each
cow to come closer to the yield that her genes make possible.
Breeding for high milk yield has brought problems. Many high-yielding
cows become lame, suffer infections of the udder called mastitis, or
struggle to become pregnant. Producing so much milk puts great demands
on the cow’s body. Because many farms use semen from the same few
bulls, much of the genetic variation in the breed has been lost.
Some farmers now choose bulls for health and long life as well as
milk. Others keep older, hardier breeds that produce less milk but
need less expensive food.
Figure 1 Mean milk yield per cow per year in one country
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term selective breeding.
(1)Describe how a farmer could use selective breeding to increase the milk yield of a herd.
(3)Use Figure 1 to calculate the percentage increase in mean milk yield between 1960 and 2020. Give your answer to the nearest whole number.
(2)Explain why milk yield shows continuous variation.
(2)Suggest why the increase in milk yield shown in Figure 1 cannot all be caused by selective breeding.
(2)Explain why the loss of genetic variation in a breed of cattle could be a problem in the future.
(3)Total for this question: 13 marks
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