عنوان مقاله [English]
Background and objectives: In previous studies in low-protein diets, mostly used pure sources of protected methionine or combined lysine and methionine used. It has been used in limited studies of the pure source of protected lysine to balance the dietary amino acids. It seems that investigating the effect of protected lysine and methionine supplementation alone and in combination, in low-protein diets, especially in early lactation, is useful to achieve maximum performance. Therefore, the purpose of this study was to investigate the effects of reducing dietary crude protein (CP) concentration and alone and in combination use of protected lysine and methionine amino acids on milk yield and milk composition and concentration of blood chemical metabolites in Holstein lactating cows during early lactation.
Materials and methods: In this study, 8 Holstein lactating cows with average 89±37 days in milk were used in a repeated measurement design two 21 day periods; each period contains 17 day for adaptation and 4 day for sampling. Treatments were: 1) a formulated diet with 17.8% CP, 2) a formulated diet with 16.5% CP plus 6-gram per day protected lysine, 3) a formulated diet with 16.5% CP plus 13-gram head per day protected methionine, 4) a formulated diet with 16.5% CP plus 6-gram lysine and 13-gram protected methionine per head day.
Results: The results of this study showed that dry matter intake, the percentage of milk fat and protein, molar proportion of volatile fatty acids, pH of rumen fluid and apparent digestibility of nutrients were not affected by experimental treatments. (P > 0.05). The results of independent comparisons in milk yield and feed efficiency indicated that by reducing the dietary protein concentration and the addition of protected methionine (alone and in combination with lysine) led to significantly increased in milk yield and feed efficiency, relative to the control group (P < 0.05). But in independent comparisons inside the treatments there was no significant difference between protected lysine and methionine in milk production and feed efficiency (P > 0.05). The addition protected lysine and methionine, alone and in combination resulted in a significant decrease (P < 0.05) in the milk concentration of urea nitrogen, relative to the control (respectively 12/36, 12/74, 13/12, to 15/37). Crude protein intake in the supplemented protected methionine and lysine, alone and in combination, group compared to the control group was significantly decreased (P < 0.05). Methionine and lysine supplementation (alone and in combination) resulted in an increase (P < 0.05) in the serum albumin total protein concentration, compared to the control group. In cattle receiving protected methionine serum urea nitrogen concentration was lower than that of control (P < 0.05). The addition of protected methionine (alone and in combination with lysine) led to significantly increase (P < 0.05) in serum glucose concentration in early lactation.
Conclusion: The results of this study indicate that by decreasing the amount of protein in the diet from 17.8% to 16.5% in early lactation and the use of protected amino acids without adversely affecting on the nutrients. Also, the results of the present study showed that combined use of protected lysine and methionine compared to their individual use, one more effective to improve the performance of Holstein lactating cows during early lactation.