Effects of Heat Shock Protein-70 Gene and Forage System on Milk Yield and Composition of Beef Cattle1

Genomic DNA from 117 Angus, Brahman, and reciprocal-cross cows was used to determine the influence of heat shock protein-70 haplotype and forage type [endophyte-infected tall fescue (Neotyphodium coenophialum; E +) or common bermudagrass (Cynodon dactylon; BG)] on milk yield and milk composition (protein, fat, and SCC). Blood samples (n = 117) were collected, the buffy coat was separated, and genomic DNA was extracted. A 523-bp fragment of the heat shock protein-70 gene was amplified by PCR, purified, and sequenced to determine the single nucleotide polymorphisms (SNP). Two of 8 previously determined SNP (at base 1,926 as a C > G base substitution with a frequency of 3.8% and at base 2,033 as a G > C base substitution with a frequency of 14%) were found to be functional and in the coding region. Cows were grouped based on predetermined SNP profiles as haplotype 1 [24 Angus (AA), 23 Brahman (BB), and 45 crosses (AB or BA)], haplotype 2 [8 AA, 5 BB, and 6 AB or BA], or haplotype 3 [2 AA and 4 AB or BA]. Milk yield and composition were determined on 5 dates in each of 3 yr during the grazing period (May through September). Mean milk yield was greater (P < 0.01) for cows grazing BG (average 5.2 ± 0.06 kg/d) compared with cows grazing E + (average 3.7 ± 0.23 kg/d). Similarly, milk fat content was greater (P < 0.01) for cows grazing BG (average 3.93 ± 0.21%) than for cows grazing E + (average 3.20 ± 0.26%). Our data indicated that tendencies existed for a haplotype × forage interaction on milk yield (P = 0.10) and a haplotype × month interaction for SCC (P < 0.05).