Comparison of milking characteristics and feed conversion efficiency of two lines of Holstein-Friesian cows which differ genetically in live weight : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Dairy Production at Massey University, Palmerston North, New Zealand
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Date
2002
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Massey University
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Abstract
Milking characteristics during peak yield in two consecutive lactations (seasons 2000 and 2001, experiment one); daily milk production and composition, somatic cell count, live weight and body
condition score during a complete lactation (2000 season, experiment two); and metabolisable energy
intake and feed conversion efficiency during peak lactation (1999 season, experiment three) were
studied in three experiments with grazing Holstein-Friesian cows from two selection lines, which
differed genetically for live weight. Experiment one (a & b): the heavy line yielded more milk at each
milking than the light line but this difference was not significant for any season. Average flow rates
were similar for both lines in both lactations (~2.0 litres/min for both lines). Maximum flow rates did
not differ between lines either (~3.2 litres/min for both lines). Consequently, total milking times were
similar for both lines in both lactations (7.5 vs. 7.3 min and 7.6 vs. 7.8 min for the heavy and the light
line for seasons 2000 and 2001 respectively). Experiment two: Cows from the heavy and the light line
yielded 22.2 and 20.6 litres/day respectively (p<0.01). Fat yield was similar for both lines because the
milk from the light cows had a higher fat concentration than milk from the heavy (4.8 vs. 5.0%;
p<0.05). The heavy line yielded more milk protein than the light line (0.8 vs. 0.7 kg/day; p<0.05),
however, there were no significant differences between lines for protein concentration. Log
transformed milk somatic cell counts were slightly lower for the heavy line both in peak lactation and
during the whole lactation, however, this difference was significant only during peak lactation in 2001
(10.8 vs. 11.4x103 cells/ml of milk, p<0.001; and 10.3 vs. 10.8x103 cells/ml of milk, p<0.05 for the
heavy and light line for period one and two respectively). Differences in live weight between the
heavy and the light line were significant (517 vs. 474 kg for the heavy and the light line respectively;
p<0.001). Body condition score during the whole lactation was similar for both lines (4.2). Experiment
three: metabolisable energy intake and feed conversion efficiency in peak lactation were similar for
both lines (158 vs. 161 MJ ME/cow/day and 108 vs. 106 g MS/kg DM intake for the heavy and the
light line respectively). The regression coefficient of metabolisable energy intake on metabolic live
weight was 0.65 MJME/kg LW0.75 for both lines. In summary, selection for cow live weight affected
the live weight of the cows, had no effect on milk production, and in contrast with other experiments,
had no effect on individual pasture intake either per cow or per kg of metabolic live weight nor on
energy requirements for maintenance. Finally, selection for cow live weight did not have a consistent
effect on milking characteristics or milk somatic cell counts.
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Keywords
Milk yield, Milk production, Dairy cows, Somatic cell counts, Feed efficiency, Holstein-Friesian cows, Live weight