Absorbed AA can be used for maintenance, growth, pregnancy, and milk yield. We hypothesized that certain AA may also affect milk fat yield in dairy cows. The objective was to evaluate the effects of individual absorbed amino acids (AA) on milk fat yield of lactating dairy cows. A database of 175 studies (724 treatment means) conducted with dairy cows (BW = 604 ± 46 kg, days in milk = 135 ± 53.6) fed diets containing 50.9 ± 10.2% forage (starch = 27.7 ± 6.96, neutral detergent fiber = 32.1 ± 5.50, crude protein = 16.5 ± 2.502, rumen degradable protein = 11.39 ± 1.725, % DM) was used. Dietary composition of each AA was calculated using the Cornell Net Carbohydrate and Protein System feed library. The absorbed amount of each AA (g/d) was calculated using equations for estimating 1. AA supplied by the microbial protein using rumen degradable protein, 2. AA supplied by rumen undegradable protein, and 3. the intestinal digestibility of each AA (Myers et al., 2019). Statistical analysis was conducted using R (v. 3.5.1, 2019). A linear mixed model was fitted (lme4) using a random effect of study and weighted using the square root of the number of animals represented in each treatment. Milk fat was positively related to absorbed Met and Ile yielding more than 2 g/d of milk fat per g of absorbed AA, while absorbed Lys was associated with a milk fat response of 0.9 g/d of milk fat per g of Lys (P < 0.05). No other essential AA evaluated (Arg, His, Leu, Phe, Thr, Trp, Val) significantly affected milk fat yield (Table 1). Our results showed that absorbed Met, Lys, and Ile should be considered as potential drivers of milk fat yield in dairy cows. Table 1 (Abstr. M191).

Predictor (g/d)	Estimate	SE	95% CI	P-value
Intercept	426.251	43.348	341.29–511.21	<0.001
Absorbed Arg	−0.482	0.884	−2.22–1.24	0.580
Absorbed His	−2.127	1.571	−5.21–0.95	0.176
Absorbed Ile	2.185	0.962	0.30–4.07	0.024
Absorbed Leu	−0.538	0.361	−1.25–0.17	0.137
Absorbed Lys	0.927	0.469	0.01–1.85	0.049
Absorbed Met	2.066	0.628	0.84–3.30	0.001
Absorbed Phe	−0.388	0.709	−1.78–1.00	0.585
Absorbed Thr	2.003	1.360	−0.66–4.67	0.141
Absorbed Trp	6.034	4.223	−2.24–14.31	0.154
Absorbed Val	0.695	1.348	−1.95–3.34	0.606