Effects of rumen-protected lysine and methionine supplementation in low-crude protein diets on lactation performance, nitrogen metabolism, rumen fermentation, and blood metabolites in Holstein cows

Abstract

The objective of the study was to investigate the effects of supplementing a low-protein diet with rumen-protected lysine (RP-Lys) and RP-methionine (RP-Met) on milk production, nitrogen metabolism, and rumen fermentation in dairy cows. Forty-eight Holstein dairy cows were blocked by parity (2.1 ± 0.17), milk production (32.1 ± 3.41 kg/d), and days in milk (124 ± 28 d), and randomly assigned to 1 of 4 dietary treatments: control diet (CON, 16 % CP), low-protein diet (LP, 14 % CP, negative control), a moderate diet supplemented with RP-Met (31 g/d) and RP-Lys (MPML, 15 % CP, 110 g/d), and LP diet with RP-Met (50 g/d) and RP-Lys (LPML, 14 % CP, 150 g/d). Dry matter intake (DMI), feed efficiency, and the apparent total tract digestibility of nutrients was not affected by treatments. Compared with the CON, the milk yield was increased in the MPML treatment (P = 0.03) while decreased in the LP treatment (P = 0.02). Milk protein yield in the MPML treatment was higher than that in the other treatments (P < 0.05). The LP, LPML, and MPML treatments had lower concentrations of milk urea-N and plasma urea-N than CON treatments (P < 0.01). The LP, LPML, and MPML treatments decreased the plasma histidine (His) concentration (P < 0.01), while the LP and LPML treatments decreased plasma arginine (Arg) concentration (P < 0.05) compared to the CON treatment. Plasma Met concentration in the MPML treatment was increased (P < 0.01), whereas was decreased in the LP treatment, compared with the CON (P < 0.05). The N intake (g/d) in the LP, LPML, and MPML treatments was lower than that in the CON treatment (P < 0.01), while fecal N (g/d) was not affected by treatments. However, CON treatment increased the urine N (g/d), total excreta N (g/d), N secretion and excretion (g/d), and the proportions of urine N to N intake, whereas decreased the proportion of milk N to N intake and milk N efficiency, compared to the other three treatments (P < 0.01). The rumen concentration of ammonia nitrogen in the LP, LPML, and MPML treatments was lower than that in the CON treatment (P < 0.05). Collectively, the low-protein diets supplemented with RP-Met and RP-Lys could increase milk N efficiency and dramatically decrease urinary N losses. From the perspective of N emission reduction, a 2% reduction in dietary CP was rational.