Abstract
Key words
Introduction
Materials and Methods
Data
Category | Parity | |||
---|---|---|---|---|
1 | 2 | 3 | All | |
Herd–test month–test year classes, n | 9,039 | 6,170 | 4,827 | 9,039 |
Cows, n | 25,609 | 13,195 | 8,064 | 25,609 |
Test-day records, n | ||||
<87.5% Holstein genetics | 64,550 | 43,194 | 28,024 | 135,768 |
87.5 to 93.6% Holstein genetics | 84,309 | 38,358 | 23,493 | 146,160 |
≥93.7% Holstein genetics | 69,224 | 30,930 | 18,656 | 118,810 |
Total | 218,083 | 112,482 | 70,173 | 400,738 |
Mean test-day milk yield ± SD, kg | ||||
<87.5% Holstein genetics | 11.9 ± 4.4 | 13.1 ± 5.1 | 13.9 ± 5.5 | 12.7 ± 4.9 |
87.5 to 93.6% Holstein genetics | 12.9 ± 4.8 | 14.6 ± 5.8 | 15.6 ± 6.4 | 13.8 ± 5.4 |
≥93.7% Holstein genetics | 14.2 ± 5.5 | 15.6 ± 6.5 | 16.4 ± 7.0 | 14.9 ± 6.1 |
Mean DIM ± SD, d | ||||
<87.5% Holstein genetics | 148 ± 83 | 144 ± 82 | 143 ± 81 | 146 ± 81 |
87.5 to 93.6% Holstein genetics | 147 ± 83 | 147 ± 83 | 146 ± 83 | 147 ± 83 |
≥93.7% Holstein genetics | 148 ± 84 | 148 ± 84 | 147 ± 84 | 147 ± 83 |
where T is temperature in degrees Celsius and RH is relative humidity as a percentage. Mean daily THI 3 d before each milk test date was used to detect the threshold of heat stress and to estimate genetic parameters as suggested by
Model
where yijklmn is test-day milk yield of cow n in herd–test month–test year (HMY) class i within parity j (1, 2, or 3), DIM class k (5 to 34, 35 to 64, …, 275 to 305 DIM) nested within breed group (BG) class l (<87.5, 87.5 to 93.6, or ≥93.7% Holstein genetics) and parity j, and calving age (CA) class m (CA was grouped every 2 mo for each parity) within parity j; HMY, BG, DIM, and CA are fixed effects; a is random additive genetic effect without consideration of heat stress (regular); α is random additive genetic effect of heat stress; p is a random permanent environmental effect without consideration of heat stress (regular); π is random permanent environmental effect of heat stress (HS); e is random residual effect; and f(THI) is a function of THI:
Results and Discussion
Climatic Conditions in Thailand


Determination of Heat-Stress Threshold

Genetic Estimates
Parameter 1 Random additive genetic effects for regular and heat stress are denoted by a and α, respectively; permanent environmental effects for regular and heat stress are denoted by p and π, respectively; σa2 = additive genetic variance without consideration of heat stress; 100(σα2) = additive genetic variance for heat stress at a THI of 90 (10°C over a THI threshold of 80); 10(σaα) = additive genetic covariance between effects with and without heat stress considered; σp2 = permanent environmental variance without consideration of heat stress; 100(σπ2) = permanent environmental variance for heat stress at THI of 90; 10(σpπ) = permanent environmental covariance between effects with and without heat stress considered; σe2 = residual variance; rg=genetic correlation; and rp=permanent environmental correlation. | Parity | ||
---|---|---|---|
1 | 2 | 3 | |
3.04 ± 0.21 | 4.86 ± 0.35 | 6.30 ± 0.55 | |
0.27 ± 0.12 | 0.64 ± 0.29 | 1.22 ± 0.41 | |
−0.19 ± 0.16 | −0.59 ± 0.31 | −0.85 ± 0.44 | |
7.91 ± 0.19 | 10.21 ± 0.31 | 13.06 ± 0.49 | |
12.08 ± 0.28 | 19.22 ± 0.57 | 22.60 ± 0.88 | |
−4.43 ± 0.18 | −7.23 ± 0.34 | −9.69 ± 0.52 | |
5.69 ± 0.02 | 7.72 ± 0.04 | 9.39 ± 0.06 | |
rg | |||
a (parity 1, parity ≥1) | 0.94 | 0.88 | |
a (parity 2, parity 3) | 0.96 | ||
α (parity 1, parity ≥1) | 0.22 | 0.08 | |
α (parity 2, parity 3) | 0.14 | ||
a, α | −0.21 | −0.33 | −0.31 |
rp | |||
p (parity 1, parity ≥1) | 0.32 | 0.24 | |
p (parity 2, parity 3) | 0.30 | ||
π (parity 1, parity ≥1) | 0.15 | 0.09 | |
π (parity 2, parity 3) | 0.24 | ||
p, π | −0.45 | −0.52 | −0.56 |
Permanent Environment Estimates
Correlations Between Effects With and Without Heat Stress Considered
Conclusions
Acknowledgments
References
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