Publication: Frontiers | Milk Composition for Admixed Dairy Cattle in Tanzania
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2018-04-24
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Frontiers Media
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Abstract
It is well established that milk composition is affected by the breed and genotype of a
cow. The present study investigated the relationship between the proportion of exotic
genes and milk composition in Tanzanian crossbred dairy cows. Milk samples were
collected from 209 animals kept under smallholder production systems in Rungwe
and Lushoto districts of Tanzania. The milk samples were analyzed for the content of
components including fat, protein, casein, lactose, solids-not-fat (SNF), and the total
solids (TS) through infrared spectroscopy using Milko-Scan FT1 analyzer (Foss Electric,
Denmark). Hair samples for DNA analysis were collected from individual cows and breed
composition determined using 150,000 single nucleotide polymorphism (SNP) markers.
Cows were grouped into four genetic classes based on the proportion of exotic genes
present: 25–49, 50–74, 75–84, and >84%, to mimic a backcross to indigenous zebu
breed, F1, F2, and F3 crosses, respectively. The breed types were defined based
on international commercial dairy breeds as follows: RG (Norwegian Red X Friesian,
Norwegian Red X Guernsey, and Norwegian Red X Jersey crosses); RH (Norwegian
Red X Holstein crosses); RZ (Norwegian Red X Zebu and Norwegian Red X N’Dama
crosses); and ZR (Zebu X GIR, Zebu X Norwegian Red, and Zebu X Holstein crosses).
Results obtained indicate low variation in milk composition traits between genetic groups
and breed types. For all the milk traits except milk total protein and casein content, no
significant differences (p < 0.05) were observed among genetic groups. Protein content
was significantly (p < 0.05) higher for genetic group 75–84% at 3.4 ± 0.08% compared
to 3.18 ± 0.07% for genetic group >84%. Casein content was significantly lower for
genetic group >84% (2.98 ± 0.05%) compared to 3.18 ± 0.09 and 3.16 ± 0.06%
for genetic group 25–49 and 75–84%, respectively (p < 0.05). There was no significant
difference (p < 0.05) between breed types with respect to milk composition traits. These
results suggest that selection of breed types to be used in smallholder systems need not
pay much emphasis on milk quality differences as most admixed animals would have
similar milk composition profiles. However, a larger sample size would be required to
quantify any meaningful differences between groups.
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Keywords
Milk Composition, Breed Type, Genetic Group, Genomic Markers, SNP, Crossbred Cows, Tanzania