to ensure that they can respond to their surroundings; thus, it is advantageous to have several protein mechanisms, i.e. at higher brain, hypothalamus, pituitary and target gland levels. However,
when pushed too far, subfertility occurs. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Stress; Reproduction; Subfertility
1. Introduction
‘‘Stress’’ is responsible for many things, including subfertility. Many agricultural advisers and veterinarians are very familiar with those intangible factors that reduce
fertility on farms but often they are unable to pinpoint precise contributory causes — and hence blame ‘‘stress’’.
This, in itself, provides a definition of ‘‘stress’’, that is, the inability of an animal to cope with its environment, a phenomenon that is revealed by a failure to achieve genetic
potential, e.g. for growth rate, milk yield, disease resistance, or fertility.
2. Field observations
Strong evidence that stressors affect reproductive efficiency in dairy cattle has been gained by comparing fertility data of normal cows and herd-mates suffering from
Ž .
various stressful clinical conditions Table 1 . Furthermore, evidence of a social stressor affecting fertility has been provided by a
behavioural study, which identified cows that changed social position in the herd hierarchy within the breeding period. Those cows that increased social status were more
fertile and had better milk production figures than those with a lowering of social status Ž
. Table 2; Dobson, unpublished data . They also had a different lameness score on a 0–5
point scale, with 5.0 representing a very lame animal. Unfortunately, the clinical conditions in Table 1 and social interactions exemplified
in Table 2 are very common in the dairy industry. Worse than that, these factors are
Table 1 Summary of fertility parameters of dairy cows with clinical disease conditions diagnosed and treated in the
Ž .
postpartum period data selected from Borsberry and Dobson, 1989; Collick et al., 1989 . Each diseased cow was compared with a similar untreated healthy herd-mate
Ž .
Ž .
Pairs Ca — 1st service days
Ca — conception days Inseminations per conception
Control Diseased
Control Diseased
Control Diseased
Milk fever 95
61 67
75 88
1.2 1.7
Ovarian cyst 73
63 78
77 142
1.3 3.2
Sick 9
72 69
76 157
1.3 3.1
Lameness 427
68 72
86 100
1.7 2.1
P - 0.05. P - 0.01.
P - 0.001.
Table 2 Summary of fertility and milk production figures for 45 pairs of cows that displayed increasing or decreasing
Ž .
social status based on dominance and submissiveness during the breeding period in three commercial dairy herds
Change in social status Increase
Decrease Ž
. Calving to conception days
97 143
Inseminations per conception 1.6
2.2 Ž
. Milk yield kgrday
q0.58 y1.03
Ž .
Somatic cell counts ’000rml y18
q371 Difference in lameness score
y0.21 q0.54
P - 0.05.
clearly hindering the genetic progress of one of the major domesticated species in the world. No doubt similar data can be compiled for other commercially important species.
If we are to avoid paying this price for domestication of any species, it is necessary to learn more about how animals respond to stressors, and how this affects the mechanisms
controlling reproductive efficiency.
3. Examination of control mechanisms
