Genetics of dairy production

Suppose that we have to select 10 bulls among those 40 candidates. We can choose to select the 10 best bulls on milk yield or the 10 best bulls on protein yields or the 10 best bulls for a combination between milk yield and protein content.

Question

For the first strategy, the differential of selection will be: (?) kg for milk yield and (?) g/kg for protein content.

Solution

For the first strategy, the differential of selection will be: +785.5 kg for milk yield and -0.875 g/kg for protein content.

Here are the bulls selected:

Milk

Protein content

36

1130

-1.9

40

1080

-2.2

38

950

-2.8

35

800

-1.1

34

765

2.5

33

755

-1.15

32

625

-0.2

30

600

-0.1

31

590

-0.6

29

560

-0.5

Question

For the second strategy, the differential of selection will be: (?) kg for milk yield and (?) g/kg for protein content.

Solution

For the second strategy, the differential of selection will be: -27 kg for milk yield and +1.7 g/kg for protein content.

Here are the bulls selected:

Milk

Protein content

19

45

2.5

1

-750

2.2

21

100

2

35

800

1.8

5

-400

1.7

3

-500

1.7

4

-465

1.4

27

550

1.3

37

320

1.2

18

30

1.2

Question

Finally, for the third strategy, the differential of selection will be: (?) kg for milk yield and (?) g/kg for protein content.

Solution

Finally, for the third strategy, the differential of selection will be: +305 kg for milk yield and +1.2 g/kg for protein content.

Here are the bulls selected:

Milk

Protein content

18

30

1.2

19

45

2.5

20

45

1

21

100

2

22

370

1.1

26

530

0.4

27

550

1.3

35

800

1.8

37

320

1.2

39

260

-0.05

Fundamental

What is important to remember from this example is that:

  1. When we only consider one trait in selection, traits genetically negatively correlated will be deteriorated.

  2. When we consider both traits, we will be able to have genetic gain for both traits. However, the gain for each trait will be lower than when we only considered one trait.

  3. For traits with lower heritability, improvement of negatively correlated traits will be even more complex.

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