The Australian State of Tasmania enjoys a cool, temperate climate that
remains the backbone of its pasture-based dairy production system. In
this study, 330,366 lactation records from 428 Tasmanian dairy herds
collected between 2000 and 2005 were analysed. The objective was
to determine the inuence of genetic and non-genetic factors on milk,
protein and fat yields of pasture-based dairy cows. The data were
statistically subjected to analyses of variance using general linear
mixed model procedures with repeated measures. State-wide average
milk yield per lactation over a standard 305-day lactation length was
5200.7 ± 1239.7 litres (ranging from 1107 to 13256 litres), while fat
and protein yields averaged 205.5 ± 47.0 kg (ranging from 53 to 385
kg) and 166.2 ± 41.5 kg (ranging from 47 to 297 kg), respectively.
Highly signicant (P<0.001) effects on milk, protein and fat yields
attributable to variation in herd size, cow’s parity, breed, season and
year of calving were detected. Milk yield increased linearly with
increase in parity (means of 3482.4, 4019.5, 4615.4, 4826.1 and
5018.8 litres per lactation for parities 1, 2, 3, 4 and >4, respectively).
Milk, fat and protein yields were highest in cows calving during
the spring season (4769.8 litres, 215.2kg and 168 kg respectively),
Holstein-Friesian genotypes produced the most milk (5211 litres),
protein (171 kg) and fat (210kg) yields per lactation. Herd sizes of more
than 1110 cows produced the most milk, fat and protein. Productivity
per cow increased with calving year except in 2003 when total milk
yield was lower than in 2002. We conclude that herd size, breed,
parity, season and year of calving were among the main factors
driving production of dairy cows in Tasmania and adjustments for
these factors would be mandatory for any unbiased comparison
of lactation performance within and between pasture-based dairy
production systems
