<p class="first" id="d4845525e137">Wheat is the most common concentrate fed to grazing
dairy cows in Australia, but no
studies have examined the effects of wheat proportion in a pasture-based diet on milk
production and methane emissions. In this 47-d experiment, 32 Holstein dairy cows
were offered 1 of 4 diets during d 1 to 36. Cows in each of the dietary treatment
groups were individually offered no wheat (W0) or wheat at 3 kg of dry matter (DM)/d
(W3), 6 kg of DM/d (W6), or 9 kg of DM/d (W9). The remainder of the diet was 2.2 kg
of DM of concentrate mix and freshly harvested perennial ryegrass (Lolium perenne)
such that all individual cows were offered a total diet of approximately 20.2 kg of
DM/d. From d 37 to 47 the diets of cows receiving treatments W0 and W3 remained unchanged,
but cows in treatments W6 and W9 received the W3 diet. Individual cow feed intakes,
milk yields, milk compositions, and methane emissions were measured for d 31 to 35
(period 1) and d 45 to 47 (period 2). During period 1, the mean intakes of cows offered
the W0, W3, W6, and W9 diets were 19.2, 20.4, 20.2, and 19.8 kg of DM/d. Diet caused
differences in energy-corrected milk, and means for W0, W3, W6, and W9 were 29.5,
32.4, 33.0, and 32.9 kg/d, respectively. Milk fat percentage differed with respective
means of 3.93, 3.94, 3.69, and 3.17. Diets also caused differences in methane emissions,
with means for W0, W3, W6, and W9 of 440, 431, 414, and 319 g/d. During period 1,
the cows fed the W9 diet produced less methane and had lower methane yields (g/kg
of DMI) and intensities (g/kg of energy-corrected milk) than cows fed the W3 diet.
However, in period 2 when the wheat intake of cows in the W9 treatment was reduced
to the same level as in the W3 treatment, their methane emissions, yields, and intensities
were similar to those offered the W3 treatment, yet protozoa numbers in ruminal fluid
were still much lower than those in cows offered the W3 treatment. Our research shows
that for diets based on perennial ryegrass and crushed wheat, only the diet containing
more than 30% crushed wheat resulted in substantially depressed milk fat concentration
and reduced methane emissions, methane yield, and methane intensity. Thus, although
feeding a diet with a high proportion of wheat can cause substantial methane mitigation,
it can come at the cost of depression in milk fat concentration.
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