Bulk milk urea concentrations and their relationship with cow fertility in grazing dairy herds in Southern Chile

Abstract

Milk urea determination is being used as a broad indicator of protein/energy imbalance in dairy herds. The main purpose of this study was to compare blood and bulk milk urea values in grazing herds, to evaluate their seasonal variation under South Chilean conditions, and to examine their potential relationships with herd fertility. The association between herd blood urea concentration (mean of seven lactating cows) and bulk milk urea concentration (tank containing milk from the previous 24 h) was determined in 21 dairy herds. Reference values, seasonal and herd variance, and the frequency of herds with values outside a range of 2.5 to 7.3 mmol/l were determined in bulk milk samples obtained monthly for a period of one year from 82 suppliers at two creameries located in southern Chile. Finally, bulk milk urea was measured every two weeks in samples from 24 herds, and the first service conception rate (FSCR) from 2153 dairy cows was determined. Mean bulk urea concentration was highly correlated with mean herd blood urea concentration ( r=0.95; p<0.01). Mean urea concentration in the bulk milk samples obtained during one year from 82 herds was 4.9±1.2 mmol/l, with a range of 1.5 to 11.6 mmol/l. The highest values were found during spring and the lowest values during the summer. There was a high seasonal variation (CV=13–47%) and between-herd variation (CV=20–31%). Out of a total of 984 samples, 5.4% had urea values >7.3 mmol/l and 3.8% had values <2.5 mmol/l. Of the 82 herds, 27% had values outside the reference interval (2.5–7.3 mmol/l) on two or more occasions. FSCR was lower in herds when the bulk milk urea was >7.3 mmol/l (50.7%) than in cows, where the urea concentration was <5.0 mmol/l (73.8%) at the time of insemination. The study concluded that bulk milk urea concentrations provided information similar to herd blood urea concentrations in local grazing dairy herds. There was a high frequency of herds with abnormal values, with large variations between herds and between seasons. Increased milk urea concentrations during spring were associated with lower conception rates.