Table 3 Effects of pregnancy status and number of inseminations on culling in 39,727 Finnish Ayrshire cows calving
in 1993. The model also included parity, season, and herd Pregnancy status
a
Ž .
Ž .
Lactational stage of culling day Lactational stage of conception day
Risk ratio 0–150
0–150 1.0
UUU
25.8 151–240
0–150 1.0
UUU
151–240 1.3
UUU
1.6 241–305
0–150 1.0
UUU
151–240 2.1
UUU
241–305 2.6
UUU
4.9 305
0–150 1.0
UUU
151–240 1.7
UUU
241–305 2.9
UUU
305 3.8
UUU
23.9 Number of inseminations
UUU
10.0 1
1.0
UUU
2 0.9
U
3 0.9
UU
3 0.9
a
Stage of lactation when the farmer was assumed to know when a cow was pregnant; 0 refers to open cows Ž
. i.e., cows that did not conceive at all .
U
p- 0.05.
UU
p- 0.01.
UUU
p- 0.001.
the 15 diseases studied, retained placenta was the only disorder without an effect on culling.
Effects of pregnancy status and number of inseminations on culling of Finnish Ayrshires are in Table 3. The data are expressed as risk ratios, which measure the risk of
a cow being culled depending on the time when she was known to be pregnant Ž
. compared with a reference level i.e., pregnant by day 150 after calving, RR s 1.0 . The
knowledge about a cow’s pregnancy status had a different effect on culling at different stages of lactation. The later the cow conceived, the higher was the risk of her being
culled. Cows not inseminated at all were at 10 times higher risk of being culled than cows inseminated only once. Cows inseminated more than once were slightly less likely
to be culled than cows inseminated only once.
4. Economic optimization of reproductive performance
Reproductive performance of a dairy herd affects profitability of the farm. Reproduc- Ž
tive inefficiency reduces milk yield and number of calves born i.e., replacements
. produced , and may increase the cost of veterinary services and, as shown above, it also
affects the culling rate. Insemination, treatment, and culling decisions represent an area of dairy herd management affecting profitability. To make rational decisions, the farmer
needs a valid estimate of the future profitability of each cow, accounting for factors including age, production level, lactation stage, pregnancy status, and disease history.
Ž .
Dynamic programming DP can be used for optimizing a sequence of interrelated decisions. The objective of DP models developed for optimizing breeding and replace-
ment decisions in dairy herds is to maximize the expected net present values from Ž
current and replacement cows over a given decision horizon see, e.g., DeLorenzo et al. Ž
.. Ž .
1992 . Based on net present values, decisions are made monthly to either 1 keep and Ž .
Ž . inseminate, 2 keep and not inseminate, or 3 replace a cow. If the net present value of
a cow at a certain stage is lower than that of a replacement heifer, a decision to replace her should be made. Similarly, if the net present value from a cow leaving her open is
lower than breeding her immediately, a decision should be made to breed her.
Ž .
We used a DP model, described by McCullough 1992 , to determine the structure of Ž
. an ‘‘optimal herd’’ under Finnish conditions Rajala-Schultz et al., 1999a . The calving
interval resulting from the optimal replacement and breeding decisions over a 5-year decision horizon was 363 days; however, there was considerable seasonal variation
within parities, depending on the month of calving. It was optimal to have most calvings in the fall, and to keep the calving interval of these cows approximately 1 year. We also
estimated the effects of reproductive performance on optimal breeding decisions and
Ž .
profitability of Finnish dairy herds Rajala-Schultz et al., 1999b . It was not optimal to start breeding cows calving in spring and early summer immediately after the voluntary
waiting period; instead, it was preferable to allow the calving interval to lengthen for these cows so that their next calving was in the fall. However, cows calving in the fall
should be bred immediately after the voluntary waiting period. These findings were mainly due to seasonal milk pricing and higher milk production among fall calvers in
Finland. Reproductive performance had a considerable impact on a herd’s profitability; good heat detection and conception rates provided opportunities for management
control.
5. Conclusions
