Effects of substitution of millet straw for corn silage and alfalfa hay on lactation performance, ruminal fermentation, and blood metabolites in late-lactation Holstein dairy cows

The study was conducted to evaluate the effect of replacing dietary corn silage (CS) and alfalfa hay (AH) with millet straw (MS) on the production performance and the ruminal fermentation patterns of Holstein cows in late lactation. For this purpose, forty-eight multiparous Holstein dairy cows blocked by milk yield (32 +/- 3.2 kg/d) and days in milk (179 +/- 19 d) were randomly allocated to one of four treatments. The diets were formulated with a forage-to-concentrate ratio of 48:52 [dry matter (DM) basis] and contained the same concentrate mixtures, with different forage sources (on a DM basis): (1) Control (60% CS, 40% AH, and 0% MS; on 100% roughage); (2) LMS (54% CS, 36% AH, and 10% MS), (3) MMS (48% CS, 32% AH, and 20% MS), and (4) HMS (42% CS, 28% AH, and 30% MS). The dry matter intake and milk yield were lower in the HMS diet than those in the Control, LMS, and MMS diets (P < 0.01), whereas there was no difference in the Control, LMS, and MMS diets. Compared with the Control diet, 4% FCM, milk protein yield, and N conversion ratio decreased in the MMS and HMS diets (P < 0.01), whereas no difference between the Control and LMS diets was found. Besides, cows fed the HMS diet had lower milk fat yield, lactose percentage, and feed efficiency than those in the Control and LMS diets (P < 0.05), with no difference between the Control, LMS, and MMS diets. The percentage of milk fat and protein was not affected by the diets. Compared with the Control diet, the total VFA concentration was decreased, whereas the ammonia-N concentration was increased in the HMS diet (P < 0.05), with no difference between the Control, LMS, and MMS diets. The molar proportion of acetate and the ratio of acetate to propionate were lower (P < 0.01), while the molar proportion of propionate was higher in the Control diet than that in the MMS and HMS diets (P < 0.01), with no difference between the LMS and MMS diets. Compared with the Control, the apparent digestibility of DM, CP, NDF, and ADF was decreased, while the blood urea nitrogen concentration was increased in the HMS diet (P < 0.05), with no differences in the Control, LMS, and MMS diets. Thus, it was concluded that it is feasible to use MS as a fiber source for latelactation dairy cows. However, considering the milk production performance and the feed digestibility, the replacement of CS and AH with MS should not exceed 20% of the roughage.