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Abstract. Milk production in Thailand has been growing into an important agricultural sector, but it still faces numerous difficulties in environmental constraints. The main intention of this study was to identify significant environmental effects on production and fertility traits to give advices for farm management. Additionally, adjusting records for environmental impact is essential to define appropriate models for estimation of variance components and improving selection procedures. The data consists of production and reproduction records and body measurements from 2764 Holstein upgrade cows in 252 farms. With a body weight of 415 kg and a production level of 3668 kg milk Thai Holsteins only reach approximately 60 % of performances of Holsteins in temperate zones. Percentages of Holstein genes of cows and quality of roughage sources showed a certain effect on calving interval and services per conception but not on milk performances. Despite seasonal effects were not very pronounced on milk yield, there was a strong interaction between years and calving seasons. Estimates of variance components applying REML and animal models were in the range as expected, i.e. heritabilities for production traits between 0.34 and 0.37, for fertility lower than 0.03 and for body weight 0.46. Genetic correlations between fertility and production traits were near zero. Economic weights for milk yield and calving interval were derived as first derivation of profit functions and used for selection index calculations. Suc

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Muscle fiber characteristics and their relationship to water-holding capacity of longissimus dorsi (ld) muscle were studied in Brahman (BRA) and Charolais (CHA) crossbred bulls fattened under practical farm conditions. Thirty-four BRA and 34 CHA bulls were randomly selected and slaughtered at 500, 550 and 600 kg live weight. Parameters of water-holding capacity such as drip, ageing, thawing, cooking and grilling loss were determined. Muscle fiber characteristics were conducted for muscle fiber type percentage and cross-sectional areas of slow-and fast-twitch fiber types, and correlation coefficients to water-holding capacity parameters were calculated. Results showed that CHA meat had a better water-holding capacity (less ageing, thawing and grilling loss) when compared with BRA, whereas slaughter weights had no significant effects on these parameters. Furthermore, there were no significant differences between genotypes and slaughter weights in muscle fiber type percentage and cross-sectional areas of ld muscle. Slow-and fast-twitch fiber types of all experimental groups averaged 24.4 and 75.6%, respectively. Cross-sectional areas of fast-twitch fibers had almost twice the size of slow-twitch fibers (6,721 and 3,713 μm 2 , respectively). The correlation between muscle fiber area and water-holding capacity indicated that muscles with larger fiber areas had a lower drip and ageing loss but a higher cooking and grilling loss.

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