Canola meal is the second largest protein feed in the Northern latitudes and inclusion levels
in dairy rations are expected to increase due to projected large increases in production of canola
seed in Canada. However, a recent study (Swanepoel et al. 2014) showed that even though
higher inclusions of canola meal (CM) had a positive effect on production when CM directly
substituted for high protein corn based dried distillers grains (DDG), that there was an optimum
point at 120 to 135 g/kg of diet dry matter (DM) after which animal performance seemed to
decline. Only the amino acids (AA), methionine (Met), phenylalanine (Phe) and leucine (Leu)
could have limited production based upon plasma AA concentrations at the highest CM inclusion
level. Our objective was to determine if either Met or Phe, or both, was limiting performance of
early lactation dairy cows fed a ration containing 180 g/kg of diet DM as CM, by supplementing
a calculated target of 7.5 g of intestinally absorbable Phe/cow/d and/or 8.0 g of intestinally
absorbable Met/cow/d in ruminally protected (RP) forms to four pens of ~320 early lactation
cows/pen in a 4 x 4 Latin square with 28 d experimental periods. Dry matter intake was not
affected (avg: 27.6 +/- 0.4 kg/d) by feeding either of the RP AA, or the combination.
Phenylalanine supplementation alone had no effect on milk production or composition, and body
condition score (BCS) change compared to Control. Supplemental Met alone modestly increased
(P<0.01) milk protein and fat content, while decreasing (P<0.01) milk lactose content and yield,
but with no impact on BCS change compared to Control. Combination Met and Phe
supplementation decreased milk and lactose yields, as well as lactose content (P<0.01), while
increasing milk protein content and the BCS change (P<0.01). Urine volume (avg: 16.7 +/- 0.31
L/d) and flow of microbial protein (MCP) from the rumen (avg: 2092 +/- 52.7 g CP/d) were not
affected by any treatment. Plasma Met levels increased (P<0.01) with both Met treatments and
plasma tryptophan (Trp) levels decreased (P<0.01) with both Phe treatments. However, plasma Phe levels did not change with any treatment. Results are interpreted to suggest that delivery of
Met with RP Met feeding was higher than animal requirements and caused an oversupply of Met.
Addition of Phe to the Met supplementation changed the way energy was utilized by the cows,
redirecting energy liberated by Met from milk components toward BCS gain. It remains unclear
if Phe was limiting in the Control ration or if RP Phe was not fed at high enough levels to have a
measurable response on production. However, it is clear that AA limitations, requirements and
production responses are governed by much more than plasma AA levels. Results further suggest
that AA are bioactive metabolites to the extent that they can change animal performance, even
when they are not „limiting‟ per se, and that their supplementation to practical dairy cattle diets
should be approached with extreme caution for this reason.
Keywords: Spot urine purine; Estimated microbial flow; Plasma amino acids; Protein feeding.
Abbreviations: AA, amino acid; ADF, acid detergent fiber; ADICP, AD insoluble CP; ADIN,
acid detergent insoluble N; AL, allantoin; aNDF, amylase-treated NDF; aNDFom, aNDF free of
residual ash; BCS, body condition score; BCAA, branched-chain AA; BW, body weight; CM,
canola meal; CP, crude protein; CR, creatinine; DC305, DairyComp 305 management system;
DDG, dried distillers grains; DHIA, Dairy Herd Improvement Association; DIM, days in milk;
DM, dry matter; MCP, microbial CP; NDF, neutral detergent fiber; NEL, net energy for lactation;
OM, organic matter; PD, purine derivatives; RDP, rumen degradable CP; RP, rumen protected;
SCC, somatic cell count; SG, specific gravity; TMR, total mixed ration; TP, true protein.