Feeding high-concentrate diets has the potential to
cause milk fat depression, but several studies have suggested
that dietary sugar can increase milk fat yield.
Two experiments were conducted to evaluate the ability
of dietary molasses to prevent milk fat depression
in the presence of a 65% concentrate diet. In trial 1,
molasses replaced corn grain at 0, 2.5, or 5% of diet
dry matter in diets fed to 12 second-lactation Holstein
cows (134 ± 37 d in milk) in a 3 × 3 Latin square
design. Trial 1 demonstrated that replacing up to 5%
of dietary dry matter from corn with molasses had positive
effects on de novo fatty acid synthesis, increasing
the yield of short- and medium-chain fatty acids during
diet-induced milk fat depression. Increasing inclusion
rate of molasses increased milk fat concentration, but
decreased milk yield and milk protein yield. Trial 2 used
7 ruminally cannulated, multiparous, late-lactation
Holstein cows (220 ± 18 d in milk) to evaluate effects
of dietary molasses on ruminal parameters and milk
composition, and also to assess whether increased metabolizable
protein supply would alter these responses.
Cows were randomly assigned to a dietary treatment
sequence in a crossover split plot design with 0 and 5%
molasses diets. Dietary treatments were fed for 28 d,
with 16 d for diet adaptation, and the final 12 d for 2
abomasal infusion periods in a crossover arrangement.
Abomasal infusions of water or AA (5 g of l-Met/d +
15 g of l-Lys-HCl/d + 5 g of l-His-HCl-H2O/d) were
administered 3 times daily for 5 d, with 2 d between
infusion periods. Administration of AA had no effect on
concentration or yield of any milk components. Addition
of molasses increased milk fat concentration (2.71 vs.
2.94 ± 0.21%), but had no effect on yields of milk fat or
protein. Dietary molasses decreased total volatile fatty
acid concentration (141 vs. 133 ± 4.6 mM), decreased
the molar proportion of propionate, and increased the
molar proportion of butyrate in ruminal fluid. Molasses
also increased ruminal pH (5.73 vs. 5.87 ± 0.06),