Dairy cow response to lysine and methionine when reducing metabolisable protein through soybean meal

Reducing soybean meal (SBM) in dairy diets could be a way to increase protein autonomy of French dairy farms. However, decreasing metabolisable protein supply (PDI in INRA, 2018) through SBM in corn silage based diets, that are known to be deficient in 2 amino acids (AA), lysine (Lys) and methionine (Met), decreased dry matter intake (DMI), milk yield (MY) and protein yield (MPY). The effect of balancing Lys (AjiPro®-L) and Met (Smartamine M®) supplies at Low (LP) or High (HP) levels of Protein (124 vs 148 g/kg DM of CP; 84.7 vs 94.2 g/kg DM of PDI) were investigated according a 2×2 factorial design on 40 Holstein dairy cows at 84±20 days in milk. They were assigned to a randomised complete block design in 4 groups of 10 cows for 5 weeks. Four corn silage diets (58.7% of DM) at 1.44 MCal/kg DM of net energy were offered ad libitum in which the SBM were incorporated at 2.5 vs 8.9% in HP vs LP diets, respectively. The levels of Lys and Met in % of PDI were at 6.1 and 2.0 in unbalanced AA (LM-) and at 7.0 and 2.3 in balanced AA (LM+) diets, respectively. Results were submitted to variance covariance analyses using the MIXED procedures of SAS including the effects of parity, PDI, AA profiles and interactions on the mean of weeks 3 to 5 (n=37). Increasing Lys and Met supply tended (P<0.1) to increase DMI (24.3 vs 24.9 kg of DM in LM- vs LM+ diets). Milk protein yield were at 1,032, 1,116, 1,155, 1,151 g/d and MY were at 34.7, 36.8, 37.5, 37.1 kg/d in LPLM-, LPLM+, HPLM- and HPLM+, respectively. They both decreased with decreasing PDI supply (P<0.01) and increased in response to AA only at LP diets (P × AA interaction: P<0.03). On a repeated measurements analysis, MY tended to increase with AA supplementation only in multiparous cows (40.3 vs 40.8 kg/d; n=6 per diet). Body weight tended to be lower at LPLM- (677 kg; P × AA: P=0.06) than in other diets (from 687, 693, 690 kg in LPLM+, HPLM-, HPLM+, respectively). This study confirmed that better balancing Lys and Met in low PDI diets with low SBM supply increased dairy cows’ performances.