Date of Award
Doctor of Philosophy
Records of yield and breeding information for cows in the northeast part of the United States were analyzed to investigate the influence of days open present lactation, days open previous lactation, and days dry previous lactation on measures of lactation yield. Measures of yield were fat-corrected milk, milk, and milkfat all adjusted to a 305-day, mature-equivalent basis. Analyses were conducted on first parity records (84,356), first and second parities records (140,233) and first, second, and third parities records (174,251). The effects of previous days open, previous days dry and present days open were fit simultaneously. Best Linear Unbiased Estimate solutions were obtained;As present days open increased from 20 to 300 days open, lactation yields for fat-corrected milk, milk, and milkfat increased approximately 1250, 1350, and 45 kg. As previous days open increased from 20 to 300 days open, lactation yields for fat-corrected milk, milk, and milkfat increased approximately 625, 650, and 25 kg. Cows dry 60 to 69 days gave the most milk the following lactation. Cows dry 40 days or less produced much less the next lactation. All three measures of yield responded similarly to the effects of previous days open, previous days dry, and present days open;Multiparity analyses were conducted using a model where later parity records could be compared with more unselected first parity records. Adding additional parity information using this model made little difference in how cows responded to previous days dry and previous and present days open;Heritability estimates for previous days dry were approximately six per cent. Days dry were largely determined by environmental influences. Since heritability estimates for previous and present days open are also quite small, adjusting lactation yield records for previous days dry and previous and present days open is warranted;Multiplicative adjustment factors were developed for fat-corrected milk, milk, and milkfat. Adjustment factors could have a large impact on genetic cow evaluations which currently do not adjust for the largely environmental effects of days open and days dry.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Dennis Alvin Funk
Funk, Dennis Alvin, "The relationship of days open present lactation, days open previous lactation, and days dry previous lactation with yield " (1983). Retrospective Theses and Dissertations. 7715.