Increases in DMI when replacing GS with CS are
commonly associated with increased energy intake and,
subsequently, increased MY, provided diet DMD is
not decreased in high-CS diets (O’Mara et al., 1998).
Although animals offered CS-based diets had greater
energy intake than those offered GS-based diets, the
observed decrease in DMD for CS-based diets (Table
2) may have decreased the potential for improvements
in MY. The lack of response of MY to supplementary
HMBi is consistent with previous observations made by
Broderick et al. (2008) and Phipps et al. (2008), suggesting
that, at the production levels reported herein,
animals did not require additional methionine in the
diet. Lysine is also an important AA for milk production
(NRC, 2001). Broderick et al. (2008) noted that
supplementation with methionine to diets containing
a higher CP (16.1%) resulted in a greater MY when
compared with lower-CP diets (14.8%), suggesting that
other AA become limiting at the lower dietary CP. Although
the diets used in the current experiment were
calculated to contain adequate lysine to support the
production levels reported herein, it is possible that
a further response to MY may have been limited by
other AA, consistent with observations of Phipps et
al. (2008). The marginal improvement in the yield of
milk solids with HMBi supplementation is a result of
numeric increases in milk fat and protein yields.