A linear reduction in DMI was detected when cows
were fed up to 30% crude glycerin. This result is in
contrast with previous findings showing no changes to
DMI or milk production of high-yield dairy cows fed up
to 15% purified glycerin (Donkin et al., 2009). The reduced
intake observed with G15 or G30 may result from
changes to the physical form of diets, because glycerin
makes the TMR become more viscous; decreasing diet
palatability because of its impurities, such as methanol
and catalysts; or from increasing concentration of salt,
mainly NaCl, which increased 500% from G0 to G30.
According to ARC (1980), increased dietary intake of
salt could be detrimental to animal health, leading to
reduced consumption or poor utilization of feed, and
damage the kidneys. The NRC (2001) reported that
maximum concentration of dietary NaCl for dairy cattle
is approximately 4% on a DM basis, corresponding to
1.6% Na. The recommendation of Na for cows in the
condition of the present study is 0.32% Na, but diets
containing 15 or 30% crude glycerin had, respectively,
0.66 and 1.2% Na. Donkin et al. (2009) and Shin et al.
(2012) reported reduced digestibility of fiber when they
fed cattle purified or crude glycerin, respectively, and
this fact could also be an explanation to the reduced
DMI observed in current study.
No reduction in milk yield was observed, thus feed efficiency
increased with dietary glycerin. The lack of response
on milk production could be partially explained
by the high SEM observed for this parameter (1.59).
The improvement in feed efficiency in 21-d periods
may be maintained by NEFA and will likely not be
continued in a longer feeding period because of lower
DMI in animals fed especially G30. Santana Junior et
al. (2013), evaluating changes in body condition and
DMI, hypothesized that feed efficiency decreases with
increasing dietary glycerin. This apparent discrepancy
may result from the different indicators evaluated in
the 2 studies: in the current study changes in milk yield
were observed but not in BCS (average 3.13, in a 1-to-5
scale).