A dairy cooperative records information from cattle on four farms. Fig. 1 shows a cow being milked by an automatic unit. The table shows the mean results fo...
Question 1 Report
A dairy cooperative records information from cattle on four farms. Fig. 1 shows a cow being milked by an automatic unit. The table shows the mean results for daughters of four different bulls. The cooperative plans to use semen from selected bulls to improve milk production in its cattle. All cows were given the same type of feed, including silage made from plants, during the study.
| Bull | Number of daughters tested | Mean milk volume / dm3 per day | Daughters treated for mastitis / % |
|---|---|---|---|
| P | 42 | 23.8 | 8 |
| Q | 39 | 25.1 | 26 |
| R | 44 | 24.6 | 6 |
| S | 41 | 22.9 | 5 |
(a) Use Table 1 to name the bull whose daughters had the greatest mean milk volume. [1]
(b) Calculate the difference in mean milk volume between the daughters of bulls Q and S. [2]
(c) Explain why bull Q may not be the best bull to use for selective breeding. [3]
(d) Use Table 1 to suggest which bull the cooperative should select. Explain your answer. [4]
(e) Describe two ways the cooperative could make this comparison more reliable. [2]
(f) Explain why farmers may select cattle for a low chance of mastitis as well as for high milk yield. [2]
Answer Details
(a) Bull Q had daughters with the greatest mean milk volume: \(25.1\text{ dm}^3\) per day. [1]
(b)
\[25.1-22.9=2.2\text{ dm}^3\text{ per day}\]
The difference is \(2.2\text{ dm}^3\) per day. [2]
(c) Although Q's daughters have the highest milk volume, they also have the highest percentage treated for mastitis, \(26\%\). Mastitis is an infection or disease of the udder. It can reduce milk production, increase veterinary and treatment costs, and harm animal welfare, so Q may not be the best breeding choice. [3]
(d) The cooperative should select bull R. Its daughters have a high mean milk volume of \(24.6\text{ dm}^3\) per day and a low mastitis-treatment percentage of \(6\%\). Among the higher-yielding bulls, this is the lowest mastitis value, giving the best balance between high milk yield and health. [4]
(e) Reliability could be improved by testing more daughters and repeating the comparison in different years or on more farms. Keeping feed and living conditions the same, and calculating means from repeat measurements, would also improve the comparison. [2]
(f) Selecting cattle with a low chance of mastitis produces healthier cows and improves welfare. It also reduces treatment costs and avoids loss of milk production caused by disease. [2]
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