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Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
1 Corresponding author: john.cole{at}ars.usda.gov
Lactation records of any reasonable length now can be
processed with the selection index method known as best prediction
(BP). Previous prediction programs were limited to the 305-d
standard used since 1935. Best prediction was implemented in 1998
to calculate lactation records in USDA genetic evaluations,
replacing the test interval method used since 1969 to calculate
lactation records. Best prediction is more complex but also more
accurate, particularly when testing is less frequent. Programs were
reorganized to output better graphics, give users simpler access to
options, and provide additional output, such as BP of daily yields.
Test-day data for 6 breeds were extracted from the national dairy
database, and lactation lengths were required to be 500 d (Ayrshire, Milking
Shorthorn) or
800 d (all
others). Average yield and SD at any day in milk (DIM) were estimated
by fitting 3-parameter Wood’s curves (milk, fat, protein) and
4-parameter exponential functions (somatic cell score) to means and
SD of 15- (
300 DIM) and
30-d (>300 DIM) intervals. Correlations among TD yields were
estimated using an autoregressive matrix to account for biological
changes and an identity matrix to model daily measurement error.
Autoregressive parameters (r) were estimated separately for first (r
= 0.998) and later parities (r = 0.995). These r values were
slightly larger than previous estimates due to the inclusion of the
identity matrix. Correlations between traits were modified so that
correlations between somatic cell score and other traits may be
nonzero. The new lactation curves and correlation functions were
validated by extracting TD data from the national database,
estimating 305-d yields using the original and new programs, and
correlating those results. Daily BP of yield were validated using
daily milk weights from on-farm meters in university research
herds. Correlations ranged from 0.900 to 0.988 for 305-d milk
yield. High correlations ranged from 0.844 to 0.988 for daily
yields, although correlations were as low as 0.015 on d 1 of
lactation, which may be due to calving-related disorders that are not
accounted for by BP. Correlations between 305-d yield calculated
using 50-d intervals from 50 to 250 DIM and 305-yield calculated
using all TD to 500 DIM increased as TD data accumulated. Many
cows can profitably produce for >305 DIM, and the revised
program provides a flexible tool to model these records.
Key Words: best prediction • milk yield • long lactation
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