Effects of Partial Mixed Rations on Production, Composition and Nutritional Value of Milk in Lactating Dairy Cows in Temperate Region of Argentina

Abstract

The aim of the study was to determine the effect of supplementing an oat winter pasture with a total mixed ration (TMR) on lactation performance of dairy cows. Nine multiparous Holstein cows were used in a 3 × 3 Latin square design with three TMR: pasture ratios at 79:21 (T1), 58:42 (T2) and 33:67 (T3) on a dry matter (DM) basis. The response to the 100% TMR diet (TMR-100) was tested using an extra period of 14 days inmediately after finishing the Latin square schedule using the nine experimental cows. Total DM intake resulted higher in TMR-100 (28.2 (kg/cow · day) and decreased by 2.64 kg/cow · day in T1, 5.02 kg in T2 and 6.68 kg in T3. Yields of milk (31.2 kg/cow·day) and fat corrected milk (26.8 kg/cow · day) was similar between T1, T2 and T3. Milk yield resulted higher in TMR-100 (32.1 kg/cow · day) compared to T2 (30.7 kg/cow · day). Milk fat concentration (g/100 g) was higher in T2 (3.13) and T3 (3.20) compared to T1 (2.96) and TMR-100 (2.85) and milk fat yield did not differ. Milk protein content resulted higher in TMR-100 and T2 averaging 3.37 g/100 g compared to T1 (3.32 g/100 g). The lowest milk protein concentration was observed in T3 (3.29 g/100 g) that resulted only different to T2. Milk protein yield did not differ between T1, T2 and T3 averaging 1.03 kg/cow · day. A lower protein yield was detected in T2 (1.03) and T3 (1.00) when compared to TMR-100 (1.07). Efficiency of milk production (kg milk/kg DM intake) resulted higher in T3 (1.42) compared to T1 (1.25). Treatments that included pasture yielded a higher efficiency of milk production when compared to TMR-100 (1.13). Plasma urea concentration resulted lower in TMR-100 (33.8 mg/dl) with no significant differences for this parameter between the other treatments. Glucose plasma levels did not differ between T1, T2 and T3 but plasma non-esterified fatty acids (NEFA) gradually increased as the proportion of pasture was enhanced according to the lower energy intake. Plasma insulin levels were higher in TMR-100 and T2 whereas concentration of somatomedins (IGF-1) remained unaffected. Grouth hormone (GH) levels and the GH/insulin ratio were highly variable and not affected by treatments. Concentration of vaccenic and conjugated linoleic acids gradually increased with the inclusion of pasture in the diet. Supplementing a winter oat pasture with TMR may be a suitable strategy to maintain milk production with a high conversion efficiency but part of the produced milk could be sustained at the expense of endogenous energy mobilization. The conditions of replacing pasture for TMR in high yielding dairy cows should be defined taking into account the depressing effect of pasture on total DM and energy intake when fresh forage is included in high proportion in the diet.