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The effect of supplementary grass silage and standard concentrate on milk fat fatty acid composition and iodine value when cows are fed a whole rapeseed-based concentrate at pasture
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2010
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E. Magowan, A.M. Fearon and D.C. Patterson. The effect of supplementary grass silage and standard concentrate on milk fat fatty acid composition and iodine value when cows are fed a whole rapeseed-based concentrate at pasture. Irish Journal of Agricultural and Food Research 49: 129–139, 2010.
Abstract
The use of grass silage and concentrates to supplement fresh grass intake is commonly
practised in dairy systems. However, the effects of such supplementation within a dietary
regime designed to produce a spreadable butter are unknown. Sixteen Holstein Friesian
cows were used in an incomplete changeover design to investigate the effect on milk fat
of supplementation with grass silage (GS) or standard concentrate (SC) when offering a
concentrate based on whole rapeseed at pasture (RC+G). A control diet of fresh grass and
standard concentrate (SC+G) was also included. Diet had no effect (P > 0.05) on milk
yield or on the lactose concentration of milk. The iodine value (IV; grams of iodine per
100 g milk fat) of milk fat with the RC+G diet was greater (43.9, P < 0.05) than with the
SC+G diet (39.9). The iodine value of milk fat was reduced (P < 0.05) when RC+G+GS
was offered (41.5 g/100g), but not when RC+G+SC was offered (43.1 g/100g), compared
with when RC+G was offered. The proportion of unsaturated fatty acids in milk fat was
higher (P < 0.05) when the RC+G diet was offered compared with either RC+G+GS
or RC+G+SC. If supplementary feedstuffs are to be used in combination with a wholerapeseed-
based concentrate and pasture, then inclusion of standard concentrate would be
preferred over grass silage because the negative impact on the iodine value of milk fat was
less. However, further research is required to investigate the effect on IV of milk fat when
a standard concentrate supplement is offered at levels that increase milk yield.