Abstract:
Even though studies supplementing Phe to dairy cattle are rare, it has been identified as
limiting in corn silage based rations, after Lys and Met, as well as being important to the
mammary gland for overall milk production. Since canola meal (CM) is low in Phe, plasma Phe
concentrations decrease as more CM is included in dairy rations. A previous study fed 7.5
g/cow/day of intestinally absorbable Phe, but results suggested that it was insufficient to support
increased milk production since it was primarily used to support increased body condition score
(BCS; Swanepoel et al., 2015). Our objective was to determine if supplementing 15 g/cow/day of
intestinally absorbable Phe in a ruminally protected (RP) form (HCMP) to a ration containing
170 g/kg CM (HCM) would support increased milk production after fulfilling its apparent 1st priority of restoring previously mobilized peptides to muscle protein synthesis, thereby regaining
animal performance possibly lost with higher dietary CM inclusion levels (i.e., 130 g/kg (LCM)
to 165 g/kg (HCM)) based upon Swanepoel et al. (2015). Ruminally protected Met (2.0
g/cow/day intestinally absorbable) was added as part of the treatment ration to HCM treatments
to avoid a possible Met limitation. The experimental design was a 3 x 3 Latin square using 3
pens of ~315 early lactation cows/pen with three 21 day periods. Dry matter (DM) intake was not
affected (avg: 27.5 ± 0.5 kg/day) by feeding RP Phe and there was no impact of treatment on
milk and component yields, except a reduced lactose content (P=0.02) with Phe addition. Even
though plasma Phe levels only differed numerically between treatments, its supplementation
resulted in energy being diverted towards BCS gain as in Swanepoel et al. (2015), but not at the
expense of milk components, suggesting that higher Phe supplementation supplied enough Phe
to replace mobilized muscle protein while maintaining milk production. The lack in change of
plasma Phe concentrations could be due to extensive catabolization by the liver or hepatic
conversion of Phe to Tyr, which is supported by the change in plasma Tyr concentrations.
Interestingly, addition of Phe to the HCM ration increased whole tract neutral- and acid detergent
fiber digestibility. Perhaps Phe released into the rumen when Phe was fed stimulated fibrolytic
bacteria through a direct impact on microbes of free Phe, which has previously been shown to
enhance growth and/or capabilities of cellulolytic bacteria. Total net energy output decreased
with HCM feeding, but was restored to the level of the LCM ration for the HCMP treatment
suggesting that further investigation to determine if an even higher Phe supplementation level
may have additional benefits on milk production may have merit.
