Exterior traits of milk-deprived and milk-fed calves as indicators for body condition or nutritional status.

The intake of sufficient milk in the postnatal age is crucial for the physical development of the calf. Farmers struggle to assess whether the calves have consumed sufficient colostrum or milk on farms with unobserved birth, in cow-bonded calf-rearing systems or early group housing with unobserved bucket feeding. The study aimed to find exterior traits that can be used to classify the body condition and nutritional status of suckling calves during wk 1 to 4 after birth. In a second dataset, threshold values for the percentage change in flank circumference in relation to the milk intake (% BW) were determined in calves aged 8 to 10 d after birth. Therefore, at first, calves (n = 85) were measured and scored on 2 farms before feeding and 1 h after voluntary milk intake in the morning. For the second dataset, 10 calves had a voluntary uptake of different milk volumes (range % BW) from d 8 to 10. Significant differences in body parameters between the age groups and sex were analyzed using a Kruskall-Wallis nonparametric ANOVA. A nonparametric receiver operating characteristic analysis (ROC) was used to derive the threshold values for the distinction between milk-deprived and milk-fed status in calves and threshold values for changes in flank circumference in relation to milk intake % BW. The intake-associated gain of the calves could be visualized by measuring and scoring the external body parameters. Sex influenced only flank circumference (milk-fed) in wk 2. Using the ROC, the flank width showed a better model quality (0.54-0.68 vs. 0.39-0.64) and true-positive rate (64%-79% vs. 47%-87%) than the flank circumference. The left or right hunger pit showed the best model quality (0.61-1.00) and excellent true-positive rates (76%-100%). The false-positive rate was in the lower range (8%-23%). Following milk intake, changes in flank circumference were more distinctly detectable at lower intake levels (below 5% of BW) than at higher levels. Within this lower range, the ROC analysis yielded excellent area under the curve values and high accuracy in distinguishing between the amount of milk consumed % BW. In contrast, as milk intake increased to 6% to 10% of BW, the diagnostic performance declined noticeably. Suitable indicators were identified to sufficiently assess the body condition and nutritional status of newborn suckling calves, which could be implemented on farms to recognize and prevent critical situations in the absence of milk intake.