Comparison of mammary amino acid utilization in dairy cows fed a corn straw or mixed forage diet
It is reported that changes in mammary amino acid (AA) utilization associate with dietary systems. In this study, we investigated AA transformation efficiency (AATF) from mammary gland to milk production in dairy cows fed different diets. Twenty-four first lactating Holstein cows were used in this study. Cows were randomly assigned to high forage diet (HF, forage : concentrate = 60 : 40) with Chinese wildrye, alfalfa hay and corn silage as the forage source or low forage diet (LF, forage : concentrate = 40 : 60) with corn straw as the forage source. This study lasted for 11 weeks with 2-week of preliminary period and 9-week of trial period. Milk samples, blood in perineal artery and jugular vein were collected on the last morning of the trial, respectively. All samples were kept at -20°C for further analysis. To estimate AATF data were fitted to the model AATF = A/(B*C), where: A (g/d) was milk AA yield; B (g/L) was the concentration difference of plasma AA in perineal artery and jugular vein; C (L/d) was blood flow volume in mammary gland. Data were analyzed by MIXED procedure (SAS 9.1). The results showed that transformation efficiency of Phe increased in cows fed HF diet (0.72 vs. 0.88, P < 0.01). We observed that transformation efficiency of Thr (0.77 vs. 0.61), Asp (16.84 vs. 7.62) and Ser (2.01 vs. 1.26) was lower in HF group (P < 0.05), and HF diet tended to have a negative effect on non-essential AA transformation efficiency (1.95 vs. 1.45, P = 0.08). However, transformation efficiency of Arg (0.35 vs. 0.34), His (1.04 vs. 1.07), Ile (0.57 vs. 0.61), Leu (0.68 vs. 0.72), Lys (0.80 vs. 0.73), Met (0.74 vs. 0.60), Val (0.64 vs. 0.67), Glu (1.81 vs. 2.06), Gly (1.06 vs. 0.99), Ala (0.71 vs. 0.70), Cys (1.09 vs. 2.64), Tyr (0.80 vs. 0.84), Pro (3.75 vs. 3.32), essential AA (0.65 vs. 0.65) and total AA (1.01 vs. 0.97) were not affected by dietary treatment (P > 0.05). These results indicated that feeding a high forage diet to cows depressed mammary utilization of some amino acids but improved phenylalanine conversion efficiency.
Keywords: diet system, mammary gland, amino acid utilization