Percentages of milk fat, lactose and protein are affected by diurnal variations in dairy goats

Abstract Text: Diurnal variations in milk synthesis in dairy goats are not known but can have important implications for nutrigenomic interventions to improve milk synthesis. The diurnal variation in milk synthesis was evaluated in 12 Saanen multiparous goats in early to mid-lactation. Six goats were treated with an intrajugular injection of 2,4-thiazolidinedione (TZD) at 1000 h, and 6 control goats received saline. Goats received an NRC-compliant diet at 0800 h. All goats received an intramammary infusion with Streptococcus uberis 10 d before the onset of treatment to induce sub-clinical mastitis in the right half of the udder, the left half being used as control. Goats were milked every 2 h from 0700 h to 1900 h. Besides milk yield, milk samples were collected for components analysis and jugular blood samples were collected for analysis of NEFA, triacylglycerol (TAG), urea (BUN), and glucose. Data were analyzed using a GLIMMIX procedure of SAS with time, treatment, and treatment x time as fixed effects for blood variables, with the addition of udder halves and relative interactions as fixed effects for milk variables. Goat was used as random effect. Mean separation was done using Tukey’s test. The TZD injection did not affect any of the measured milk variables.  SCC was not affected by time but was greater in the right vs. left udder half in both treatment groups (P < 0.05). Percent milk fat peaked at 0900 h (4.9%) and decreased afterwards. The percentages of milk protein and milk urea (MUN) peaked at 1100 h (2.6% and 28.2 mg/dL, respectively) and the percentages of lactose (4.4%) and solid nonfat (SNF; 7.8%) peaked between 1300 h and 1500 h. None of the single blood metabolites were affected by treatment or treatment x time interaction but all were greatly affected by time (P < 0.05). Glucose consistently increased until 1300 h, urea reached a peak at 1100 h, NEFA decreased until 1300 h and increased afterwards, and TAG consistently decreased throughout the day. The sum of TAG+NEFA, an index of available fatty acids, was affected by treatment (P < 0.05) with values being greater in TZD compared with control. Significant (P < 0.01) positive correlations were observed between blood glucose and percent milk lactose, NEFA and SNF, TAG and SCC, and between BUN and milk fat and MUN. Negative correlations (P < 0.05) were observed between glucose and percent milk fat, and between TAG and SNF. This data highlights the diurnal variations occurring in milk synthesis in goats, suggesting that synthesis of milk fat is not exclusively driven by the availability of fatty acids.

Keywords: diurnal variations, goat, milk synthesis