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* Animal Improvement Programs Laboratory,
Agricultural Research Service, USDA, Beltsville, MD 20705-2350 Holstein
Association USA, Inc., Brattleboro, VT 05301
1 Corresponding author: dnorman{at}aipl.arsusda.gov
Emphasis by dairy producers on various yield and fitness
traits when culling cows was documented for US Holstein calvings
since 1982. Least squares differences between cows retained for
additional parities and those culled were estimated for milk, fat,
and protein yields; somatic cell score (SCS); days open (DO);
dystocia score (DS), final score (FS), and 14 type traits. Compared
with cows culled during first lactation, superiority for
first-parity milk yield was 569 to 1,175 kg for cows with 2
lactations, 642 to 1,283 kg for cows with 2 lactations, 710 to 1,350 kg for
cows with 3 lactations, and 663 to 1,331 kg for cows with
4 lactations. Cows retained
for
2 lactations had
first-parity SCS that were 0.34 to 0.62 lower (more favorable) than
those of cows culled during first lactation; first-parity SCS
for cows retained for 3 or
4 lactations were even more favorable than those of cows
with 1 or 2 lactations. The negative genetic relationship between
yield and fertility contributed to increased DO as selection for
higher milk yield persisted across time despite considerable
preference for early conception when culling cows. In 1982, cows
retained in the herd for 2, 3, and
4 lactations conceived earlier during first lactation (19,
17, and 23 fewer DO, respectively) than those culled during first
lactation; those differences had increased to 34, 41, and 52 fewer DO
by 2000. Although DS has a negative relationship with survival,
first-parity DS were only slightly lower (by 0.10 to 0.14) for
survivors than for cows culled during first lactation. Cows
retained for
2
lactations had greater first-parity FS by 1.4 to 1.9 points than
those culled during first lactation. On a standardized basis, the
most intense selection during first lactation was for milk and
protein yields with less for fat (74 to 86% of that for milk), DO (18
to 74%), FS (22 to 38%), SCS (19 to 37%), and DS (7 to 15%).
Producers continued to emphasize the same traits when culling during
second and third lactations. Trait priority by producers during
culling could aid in setting trait emphasis when selecting bulls for
progeny test and could also be useful in developing software for
index-based culling guides.
Key Words: culling • fitness trait • yield trait • selection
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