E. Katzina
Institute of General and Experimental Biology, Siberian Branch, Russian Academy of Sciences, Ulan-Ude, 670047, Russia
Dam–daughter and dam–son regressions were used to estimate heritability of body weight in yak. Heritability estimate of birth weight was 0.27 from both analyses. Estimates among animals aged 3 months and older (up to 24 months) ranged from 0.25 to 0.38, but were higher for measurements at ages of 3, 6, 16 and 18 months which coincided with periods when the environmental conditions are optimal, i.e. summer and early autumn. There were no differences between estimates obtained from dam–daughter and dam–son data sets. Phenotypic correlations of body weight at 18 months (age at economic maturity when animals are born in previous March/April) with weights at younger ages (3 to 16 months) ranged between 0.38 and 0.87. These correlation estimates were relatively higher at 3, 6 and 16 months, and indicated an opportunity to identify selection candidates for 18 month body weight based on the weights measured early in life. Possibilities for genetic improvement of body weight in yak are discussed.
Keywords: Heritability, live weight, phenotypic correlation, yak
Body weight is a lifetime index of meat productivity of an animal. Research on the genetic parameters of body weight in yak is of particular interest because inheritance of the trait in the species has not been studied to date. This information is required to facilitate genetic improvement of body weight in yak and their hybrids. This study was undertaken to estimate the heritability of body weight in yak at birth and at subsequent ages from 3 to 24 months. Phenotypic correlations were also estimated between body weight at 18 months and those obtained at earlier ages (3–16 months).
The study was carried out in the yak herds in East Sayan area where the altitude is 1200 meters above sea level (masl). Each herd had one or two bull (s) for natural mating (Maturova and Katzina 1990). The maternal influence on body weight in descendants was determined in mother–daughter and mother–son combinations from March and April calving (n = 138) in animals ranging in age from birth to 2 years. Body weight in dams was taken at the end of fattening period. Heritability was estimated separately for dam–daughter and dam–son data sets using parent–offspring regression. Phenotypic correlation was estimated by simple correlation coefficients of the raw data.
Body weights at birth and at 3, 6, 8, 12, 16 and 18 months were all measured at different calendar months of the year. Weights at 3 and 6 (summer) and 16 and 18 (early autumn) months of age were taken in the periods coinciding with optimal environmental conditions. Heritability estimates are presented in Table 1, which also indicates calendar months during which each measurement was taken.
Table 1. Heritability estimates of body weight in yak of different ages.1
|
Age of descendants (month) |
||||||||
|
Birth |
3 | 6 | 8 | 12 | 16 | 18 | 24 | |
|
|
Calendar months during which weights were taken |
|||||||
| Animal combinations | 3–5 | 6–8 | 9 | 11–12 | 3–5 | 7–8 | 9 | 3–5 |
| Mother–daughter | 0.27 | 0.33 | 0.32 | 0.29 | 0.28 | 0.36 | 0.34 | 0.25 |
| Mother–son | 0.27 | 0.29 | 0.34 | 0.31 | 0.27 | 0.32 | 0.38 | 0.26 |
| 1. Estimates are significant at P<0.05. | ||||||||
Heritability estimates were in the medium range, but tended to be slightly higher in the 'favourable months' (summer and early autumn). This tendency for (slightly) higher estimates in favourable months was possibly because the environmental component in the phenotypic variance was higher in more favourable environments than in less favourable conditions. There was no discernibly systematic difference between heritability estimates from mother–daughter and mother–son sets of the data.
Generally, increasing the selection intensity can enhance selection response for low-heritability traits. Katzina (1990) reported that phenotypic selection of female yak for the meat yield before sexual maturation could not make desirable improvement if the intensity in terms of an expected gain of a trait was lower than 30%. However, an increased intensity by 50% in terms of additional increases of average daily milk yield by 35 g, daily weight gain by 150–200 g, and reduction of 30 days for getting 190–200 kg body weight would create better progresses. Due to the low inheritability of body weight in yak, we suggested that information from descendant generations should be referred to plan a better breeding programme. Certainly, the interaction between the genotype and environment should also be considered, especially for the data from the favourable grazing seasons.
Phenotypic correlations of body weight at 18 months of age with weights at earlier ages are summarised in Table 2 for females and males separately. Phenotypic correlations of weight at 18 months old (age at economic maturity when animals are born in March/April of the previous year) with weights at younger ages (3 to 16 months) ranged between 0.38 and 0.87. For both sexes the correlation estimates were higher at 3, 6 and 16 months of age. Although the magnitude of phenotypic correlations does not reflect the magnitude of corresponding genetic correlations, these results point to a possibility that weights recorded as early as 3 and 6 months of age could be used to make verifications about later (e.g. 18 month) weights. Therefore, evaluation of phenotypes of young animals (3 to 6 months) will help to forecast their productivity in elder age and to differentiate yak dams sooner based on performance of their relatives.
Table 2. Phenotypic correlation estimates of 18 months weight with weights at younger ages.
Groups and number |
Correlation of 18 month weight with weights at: | |||||
Birth |
3 months |
6 months |
8 months |
12 months |
16 months | |
Female (86) |
0.38 ± 0.10 |
0.75 ± 0.09 |
0.77± 0.08 |
0.38 ± 0.10 |
0.43 ± 0.10 |
0.79 ± 0.04 |
Male (52) |
0.38 ± 0.11 |
0.55 ± 0.09 |
0.72 ± 0.10 |
0.50 ± 0.12 |
0.48 ± 0.12 |
0.87± 0.07 |
The author would like to sincerely thank Alexei I. Starkov of the Institute of General and Experimental Biology for assistance in preparing this paper.
Katzina E.V. 1990. Ways of accelerating of the selection process in yak keeping. In: Development of the productive forces in Siberia. Novosibirsk, Russia. pp. 144–146. [In Russian].
Maturova E.T. and Katzina E.V. 1990. The Sayan yak. Ulan-Ude, BSC SD AS USSR. 168 pp. [In Russian].
