RESUMO
We aimed to evaluate the incidence of unstable non-acid milk (UNAM) in cows fed either sugarcane or corn silage. Second, we aimed to evaluate the effect of daily variation (d 1 to 4) and alcohol grades (72, 78, and 80%) on UNAM incidence. The experiment was conducted as a split-plot crossover design, with 2 periods and 2 roughage types (sugarcane or corn silage). Thirteen multiparous Holstein cows with an average of 281 ± 29 d in milk were randomly distributed into 2 diets. Individual blood (analysis of total proteins, albumin, urea, calcium, phosphorus, magnesium, iron, chloride, glucose, and lactate) and milk samples (analysis of protein, fat, lactose and total solids, somatic cell count, and characterization of the protein profile) were collected during the last 4 d of each period. For UNAM identification, the alcohol test was conducted in milk samples at 4°C; specifically, if the sample presented the formation of clots, this would be noted as positive for UNAM. In addition, the Dornic acidity analysis was performed in the same samples to evaluate the true milk acidity. The use of sugarcane and higher degrees of alcohol were associated with increased UNAM. We observed no daily variation in UNAM. Nevertheless, we found no roughage type effect on the variables most commonly associated with UNAM, such as changes in salts in the casein micelle and, consequently, the zeta potential and the κ-casein (CN) fraction. The Pearson correlation analysis showed that the zeta potential and the concentrations of αS2-CN, blood ionic calcium, lactate, and glucose increased as the incidence of UNAM increased, showing a positive correlation among these variables. In contrast, the concentrations of lactose, phosphorus, and potassium decreased as UNAM increased, presenting a negative correlation. This study brought important discoveries to unveil why cows manifest UNAM. For instance, higher alcohol grades and cows fed with sugarcane had increased the incidence of UNAM. Additionally, animals with a higher incidence of UNAM (sugarcane-fed cows) were related to increased ionic calcium and glucose and changes in milk protein profile, with lower levels of BSA, ß-CN, and α-lactalbumin and greater αS1-CN content, all of which were correlated with UNAM. Nonetheless, this trial also provides evidence for the need for further studies to better understand the physiological mechanisms that directly affect the stability of milk protein.