Hammond and Sanders (1923) noted that pregnancy appeared to depress milk production starting at the 20th week of gestation; this effect becomes increasingly important as the lactation continues. The impact of pregnancy is particularly important on natural (total) lactation yields but as shown by Auran (1974) 305-day lactations are also affected by variation in length of current calving interval. Schaeffer and Henderson (1972) calculated adjustment factors required to eliminate the effect of open period on the current lactation. Since open period and current calving interval are highly correlated, the adjustment factors for open period should be almost identical to those for calving interval.

A. E McClintock

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 8. Symposia (2), , 32–36, 1982
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