Research Information System
2024 American Dairy Science Association® Annual Meeting, Florida, United States Of America, 16 - 19 June 2024, vol.107, pp.207, (Summary Text)
Thermotolerance can be estimated as the slope of a reaction norm model
because it represents the performance change of cows in response to
increments in the temperature-humidity index. Such a trait is heritable
and can be used to generate breeding values for heat tolerance (HT).
Our objective was to compare the performance of Holstein cows with
different breeding values for HT. Genomic predicted transmitting abilities
(gPTA) for 2,616 cows located in hot climates of Florida, California,
and Turkey were calculated based only on their genomic information
using the single-step GBLUP method. A total of 168,439 test-day records
of these cows were available for analyses. The Wood’s model was
used to fit a lactation curve per each cow-lactation combination (4,466
observations) and calculate the milk yield adjusted to 305-d of lactation
(MY305, kg). Two general linear models (M1 and M2) that included
the effects of farm-year (25 levels), lactation number (1, 2, ≥3), genetic
group (T1 = top third, T2 = middle third, T3 = bottom third, based on
the gPTA for HT), calving season (winter, spring, summer, fall), and
the interaction between genetic group and calving season were tested.
The difference between models was that in M1 the intercept of the
reaction norm was included as linear covariate while in M2 it was not.
Significant differences (P < 0.05) between genetic groups were found
for all calving seasons except for spring in M1. Least squares means ±
SE for winter-calved cows were 12,105.5 ± 126.2, 11,995.4 ± 124.9, and
11,692.9 ± 125.9 for T1, T2, and T3, respectively. For summer-calved
cows results for T1, T2, and T3 were 11,788.9 ± 141.7, 11,860.8 ± 140.9,
and 11,583.4 ± 144.5, respectively. For fall-calved cows, results were
11,819.0 ± 128.9, 11,821.3 ± 126.8, and 11,515.4 ± 123.6 for T1, T2,
and T3, respectively. For M2, no differences between the mean performance
of T1 and T3 for any season were identified (P > 0.05). These
results suggest that the inclusion of the intercept as adjustment for the
base genetic merit for milk production is important when evaluating the utility of gPTA generated using the slope of reaction norm model
for HT in Holstein cattle.