Understanding the Key to Successful Reproduction
Pregnancy Rate / Artificial Insemination (PR/AI)
Dairy cow fertility commonly is measured by calculating the percentage of
cows that conceive after a single AI service, also known as the pregnancy rate
per artificial insemination (PR/AI). Pregnancy rate per AI in lactating dairy
cows has decreased from 66% in 1951 (Spalding et al., 1974), to about 50% in
1975 (Spalding et al., 1974; Macmillan and Watson, 1975), to about 40% in 1997
(Butler et al., 1995; Pursley et al., 1997a), whereas PR/AI in heifers has
remained at 70% during this same period (Spalding et al., 1974; Foote, 1975;
Pursley et al., 1997b). Thus, this disparity in PR/AI cannot be attributed to
differences in genetic selection or semen quality between heifers and lactating
cows, but is likely due to physiological changes or stresses associated with
increased milk production per cow that have occurred during this time.
Four general factors that determine PR/AI in a dairy
herd include: 1) cow fertility; 2) bull fertility; 3) accuracy of heats; and 4)
AI efficiency. Cow fertility refers to any cow-related factors that influence
establishment of pregnancy and include factors such as inadequate nutrition and
environmental stresses. Bull fertility refers to the quality of semen used for
AI. Accuracy of heats refers to the timing of AI relative to estrus rather than
to service rate or accuracy of heat detection (discussed in the next section).
AI efficiency refers to factors affecting pregnancy rates due to AI technique.
Table 1 illustrates the four factors that affect
fertility and how they might interact to determine PR/AI. Of these four
factors, accuracy of heats and AI efficiency can be maximized through careful
reproductive management practices. Furthermore, although fertility varies among
bulls, the fertility of semen acquired through major AI organizations is controlled
and should not limit PR/AI. Changes in bull fertility can dramatically
influence pregnancy rate per AI by reducing the ability of sperm to bind to the
oocyte, by changing the fertilization rate, or by altering later embryonic
development (Eid et al., 1994; Nadir et al., 1993). Additionally, AI at the
wrong time of the estrous cycle (inaccurate estrus detection), improper semen
handling, or improper insemination techniques can also substantially reduce
pregnancy rate per AI (Grossman et al., 1995).
Table 1. Interaction among four
factors affecting pregnancy rate per AI (PR/AI) in lactating dairy cows.
|
Hypothetical Case |
Female Fertility |
Bull Fertility |
Accuracy of Heats |
AI Efficiency |
PR/AI |
|
Heifers |
85% |
95% |
95% |
95% |
73% |
|
Lactating Cows |
50% |
95% |
90% |
95% |
41% |
|
Lactating Cows + AI problems |
50% |
95% |
90% |
50% |
21% |
The first line of Table 1 shows a situation in which all
factors are optimized when inseminating heifers. As mentioned previously,
heifer fertility is high has remained unchanged over the past 40 years. The
second line shows a situation when all factors are optimized when inseminating
lactating cows. Thus, differences in fertility between heifers and lactating
cows may account for the observed differences in PR/AI. Unfortunately, factors
that limit cow fertility are poorly understood, and cow fertility is less than
50% even on well managed farms with excellent nutritional and reproductive
management. Thus, management strategies should be developed to maximize PR/AI
while realizing that high-producing herds under excellent management will
likely not exceed a PR/AI of 50%.