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With a view to sustain high milk productivity of crossbred cows breeding policy was proposed. This policy was validated based on analysis of 1739crossbred cows. The finding of the analysis suggest to modify traditional system of inter-se mating. It is advisable not to use F1 crossbred bull. It is further advisable to upgrade F1 population with exotic bull and then after upgrade crossbred male having 0.5 proportion of X-chromosomes of exotic and remaining 0.5 X-chromosome of zebu should be used in breeding programme.
Under this breeding policy mating of F1 crossbred cows with F1 crossbred bulls has not been able to sustain milk productivity of resultant progenies in F2 onward generations. In F2 and F3 generation milk productivity is declined 19.73 and 24.98 percent respectively. Twenty three percent crossbred cows in F2 and 33 percent in F3 generation regressed back to zebu parent in term of milk productivity (Mehla et al, 1988). Continuation of unsustainability in milk productivity of crossbred cows may pose adverse affect on national milk production in near future. Hence with view to sustain milk productivity of crossbred cows at higher level it has become necessary to modify inter-se mating policy suitably, keeping in view the basic limitation underlying the role of genes located on X chromosomes, in glucose metabolism and biosynthesis of milk.
Keeping in view the presence of genes for glucose -6 phosphate dehydrogenase (G- 6PD) and phosphoglycerate kinase (PGK) on X chromosome and their role in glucose metabolism via pantose phosphate pathway and Embden Meyerhof pathway respectively, it was hypothesised that "Genes on X chromosomes may affect milk production of inter-se mated crossbred cattle". In inter-se mating system it is assumed that exotic: zebu level in resultant F2 progenies remains around 50 percent. Where as in term of X chromosomes level of zebu and exotic in F2 progenies does not remain at 50%, since in F1 crossbred bulls X chromosome is carried from zebu dam, which is further transmitted to F2 progenies. Hence in F2 progenies out two X chromosomes, one X chromosome may be of zebu and another X chromosome may contain 50 % portion of zebu and another 50 % portion of exotic, assuming that there is 50 % crossing over of X chromosomes of exotic and zebu during gametogenesis. The effect of this unequal proportion of X chromosomes of zebu and exotic in F2 progenies was analysed. The hypothesis was validated through dam-daughter comparison first lactation 305 days milk yield data of 1739 Sahiwal x Brown Swiss crossbred cows (CB) classified in to four groups i.e. 1st group CB cows produced through mating of Sahiwal cows with F1 (Sahiwal x Brown Swiss) bulls and having both X chromosomes of zebu and 25 % proportion of exotic (Brown Swiss) autosomes; 2nd group CB cows produced through mating F1 cows with F1 bulls and having 1.5 portion of zebu X chromosome and 0.5 portion of exotic X chromosome; 3rd group CB cows produced through mating of F1 cows with 75% exotic: 25% zebu crossbred bulls and having 0.5 portion of zebu and 0.5 portion of exotic on both X chromosomes; 4th group CB cows produced through mating of F1 cows with exotic ( Brown Swiss ) bulls and having 1.5 portion of exotic X chromosome and 0.5 portion of zebu X chromosome. In group 2nd, 3rd and 4th cross-bred daughters were having 25%, 50% and 75% portion of X chromosomes of exotic and remaining portion of X chromosome of zebu respectively.
First lactation average 305 days (FL -305d) milk yield Dam Daughter comparison presented in the table:
| Group | First Lactation 305-Days Milk Yield "kg" | |
|---|---|---|
| Dam | Daughter | |
| 1 | 1797 ± 38 | 1872 ± 43 |
| 2 | 2690 ± 40 | 2159 ± 55 |
| 3 | 2683 ± 41 | 2876 ± 57 |
| 4 | 2725 ± 28 | 3598 ± 36 |
The milk productivity of CB daughters in first group having both X chromosomes of zebu did not differ significantly from their zebu Dams. In group 2nd , 3~d and 4th CB daughters having 25 %, 50% and 75% portion of exotic X chromosome respectively, the milk productivity of these cows was 20%, 60% and 112% higher in comparison to zebu (Sahiwal) Grand Dams. The increase in milk production in group 2nd, 3rd and 4th may be attributed to increase in proportion of X chromosome of exotic breed. These results are in agreement to the findings of Chayabutr et. al. 2000, where these workers have reported that milk productivity of interbred 50% HF: 50% Sahiwal and 87.5% HF: 12.5% Sahiwal was 19.76 and 10.98 kg per day during early lactation. The possible reason of higher productivity of high grade HF crossbred was due to efficient utilisation of glucose -6 phosphate in lactose synthesis and pantose phosphate pathway. Whereas, in case of 50% HF and 50% Sahiwal interbred crossbred cows, glucose -6 phosphate was utilised more efficiently in Embden Meyerhof pathway, during early stage of lactation, which may be a characteristic of zebu genome.
In view of above finding it is necessary to modify traditional system of inter-se mating with a view to sustain higher productivity of crossbred cattle and it is advised not to use F 1 breeding bull in breeding programme. It will ,be more appropriate to upgrade F 1 population with exotic bulls and there after upgraded crossbred male having 0.5 proportion of X chromosome of exotic and remaining 0.5 X chromosome of zebu, should be used in breeding programme for sustaining higher milk productivity of crossbred population.
