Lowering the dietary protein content can reduce N excretions and NH3 emissions from manure and increase milk N efficiency of dairy cows. However, milk yield (MY) and composition can be compromised due to AA deficiency. Methionine and Lys are known as first limiting EAA for dairy cows, and recently His is also mentioned as limiting, especially in grass-based or low-protein diets. To examine this, a trial was conducted with a 3-wk pre-experimental adaptation period (diet 16.5% crude protein), followed by a depletion period of 4 wk, in which 39 cows (average ± standard deviation: 116 ± 29.3 d in milk, 1.8 ± 1.2 lactations, 638 ± 73.2 kg of body weight, and 32.7 ± 5.75 kg MY/d) received a low-protein diet (CTRL) (14.5% crude protein). Then, taking into account parity, His plasma concentration, and MY, cows were randomly assigned to 1 of 3 treatment groups during the rumen-protected (RP) AA period of 7 wk; (1) CTRL; (2) CTRL + RP-Met + RP-Lys (MetLys); (3) CTRL + RP-Met + RP-Lys + RP-His (MetLysHis). Products were dosed, assuming requirements for digestible (d) Met, dLys, and dHis being, respectively, 2.4%, 7.0%, and 2.4% of intestinal digestible protein. In the cross-back period of 5 wk, all cows received the CTRL diet. During the last week of each period, a N balance was conducted by collecting total urine and spot samples of feces. Total feces production was calculated using the inert marker TiO2. Statistical analysis was performed with a linear mixed model with cow as random effect and data of the last week of the pre-experimental period used as covariate for the animal performance variables. No effect of supplementing RP-Met and RP-Lys nor RP-Met, RP-Lys, and RP-His on feed intake, milk performance, or milk N efficiency was observed. However, the plasma AA profile indicated additional supply of dMet, dLys, and dHis. Nevertheless, evaluation of the AA uptake relative to the cow's requirements showed that most EAA (exclusive Arg and Thr) were limiting over the whole experiment. Only dHis was sufficiently supplemented during the RP-AA period due to an overestimation of the diet's dMet and dLys supply in the beginning of the trial. The numerically increased milk urea N and urinary N excretion when RP-Met, RP-Lys, and RP-His were added to the low-protein diet suggest an increased catabolism of the excess His.

Reduction of crude protein (CP) content in swine diets must be associated with an adequate quantitative and qualitative provision of amino acids (AA). In piglet diets, the fifth limiting AA is Val, where deficiency has been proven to reduce feed intake. In addition, depending on the overall level of CP and AA, diets high or low in standardized ileal digestible (SID) Lys will have different branched chain amino acid (BCAA) balances, which may in turn affect the feed intake response. Therefore, two Lys dose response trials (Exp. 1 and 2) were per-formed to study the influence of different AA concentrations on feed intake and overall performance. CP levels differed between the two experiments (201-210 g/kg in Exp. 1 and 180 g/kg in Exp. 2). Next, one Val dose response (Exp. 3) was carried out to establish the optimal SID Val:Lys ratio for 4 to 9-week-old Pietrain piglets. In Exp. 1, raising dietary SID Lys from 8.50 to 13.50 g/kg resulted in a significant linear and quadratic increase of average daily gain (ADG) and gain to feed (G:F) (P < 0.001 and P < 0.01 respectively), while average daily feed intake (ADFI) showed a linear increase with increasing SID Lys levels (P < 0.001), with a tendency to a quadratic effect (P = 0.08). In Exp. 2, ADG and G:F responded linearly and quadratically to SID Lys levels from 8.50 to 13.50 g/kg (P < 0.001), and ADFI showed a quadratic (P < 0.001) but not linear (P = 0.144) response to increased dietary SID Lys. For Exp. 1, the break points were above the tested SID range. For Exp. 2, quadratic polynomial (QP) and broken line quadratic (BLQ) model for ADG gave an optimal SID Lys of 12.05 and 11.39 g/ kg, respectively, the QP ADFI reached an optimum of 11.33 g/kg of SID Lys. For G:F an optimal SID Lys level of 13.23, 11.72, and 13.28 g/kg SID Lys were estimated with a QP, broken line linear (BLL), and BLQ respectively. In Exp. 3, increasing the dietary SID Val:Lys level resulted in a linear and quadratic increase of ADG and ADFI (P < 0.001), but not G:F (P = 0.626 and P = 0.114). For ADG and ADFI it was possible to fit a QP and a BLQ model, with an optimal SID Val:Lys of 0.74 for the QP and 0.68 for the BLQ model, respectively, for the two response variables. In conclusion, SID Lys requirements of 4 to 9-week-old piglets are estimated to be equal or greater than 11.4 g/kg (1.14%), and SID Val:Lys requirements were found to be between 68 and 74% for optimal growth and feed intake.

A dose-response study was performed to estimate the valine (Val) requirement in piglets between 4 and 9 weeks of age. For this end, 432 piglets (weaned at 4 weeks of age, 8.0 ± 0.9 kg) were distributed among eight diets differing in their Val to lysine (Lys) ratio expressed on a standardized ileal digestible (SID) basis. Two low crude protein diets (168 g/kg) only differing in SID Val:Lys level, one deficient in SID Val:Lys (0.58 SID Val:Lys) and the second one with a SID Val:Lys ratio expected to be above requirements (0.825 SID Val:Lys), were mixed in different proportions to obtain eight SID Val:Lys levels. The diets were formulated to be isocaloric (9.8 MJ net energy/kg) and with a sub-limiting SID Lys level (10.5 g/kg), allowing to estimate the Val requirement relative to Lys. The average daily gain (ADG) and feed intake (ADFI) were improved with increasing SID Val:Lys level; the best performance being obtained with 0.72 SID Val:Lys. Feed intake increased from 402 ± 45 g/day at 0.58 to 556 ± 42 at 0.72 Sid Val:Lys. Daily gain improved from 276 ± 37 g/day at 0.58 Sid Val:Lys to 391 ± 38 at 0.72 Sid Val:Lys. Using ADG and ADFI as response criteria, the requirement was estimated to be 0.675 and 0.742 SID Val:Lys with curvilinear-plateau and quadratic models, respectively. These results are in line with the literature and confirm that piglets react to a Val deficiency by lowering their ADFI which reduces their ADG without affecting feed efficiency. The results of the present experiment highlights that the Val level has to be controlled in piglet diets to avoid losing growth rate.

In order to fine-tune standardised ileal digestible (SID) lysine (Lys) requirement of modern piglets (RA-SE genetics hybrid sow x Pietrain boar), two dose-response studies to Lys were conducted in 4 to 9 weeks piglets (8–24 kg). In trial 1, only commercially available feed-grade amino acids (AA) were used, without maintaining dietary crude protein (CP; 201–210 g/kg). Net energy was fixed at 9.8 MJ/kg. In trial 2, all the range of feed-grade AA needed were used while dietary crude protein was kept constant at 180 g/kg by adding L-Glutamic acid with decreasing Lys levels. Net energy level decreased slightly from the lowest to the highest Lys level (from 10.1 to 9.8 MJ/kg), because of the high energy content assigned to glutamic acid. All feeds were pelleted. On both trials, SID Lys ranged from 8.5 to 13.5 g/kg, divided over 5 levels. Six (trial 1) or nine (trial 2) pens of 6 piglets were tested per level. ADG, ADFI and G:F showed clear dose-responses to supplementation of Lys in both trials. In trial 1, it was not possible to reach an optimum for any of the growth parameters using a non-linear regression model, suggesting an optimum above 13.5 g/kg SID Lys. In contrast, in trial 2, best performance was observed at 11.4 and 13.2 g/kg SID Lys for ADG and G:F, respectively using a curvilinear-plateau model. While maximal performance was similar in the two trials (G:F=0.744-0.737, ADG=450-446 g/d), the effect of Lys shortage was higher in trial 1, probably due to an imbalanced AA:Lys ratio. To the contrary in trial 2, the SID Lys:CP ratio was 0.075 at the highest Lys level, suggesting that nitrogen or non-essential AA may have become limiting.