Onset of feeding behavior in intensively housed dairy calves

Development of early feed and water intake is not well documented in dairy calves, and little is known about how traditional, intensive feeding management influences the development of species-specific behaviors, such as rumination. Our first objective was to describe grain, water, and hay intake in calves from birth. Our second objective was to evaluate whether early access to hay influenced the likelihood of reaching biologically relevant thresholds of grain and water intake (grain: 50, 100, 250, 500, 900 g; water: 1, 2, 3, 4, 5 L), and rumination (5%, 10%, 15%, 20% of a 24-h d) compared with grain alone. Our third objective was to evaluate if hay provision affected the age at which calves reached these thresholds. We enrolled 49 Holstein heifer calves over 2 yr (2018: 22; 2019: 27) that were housed individually and fed milk replacer via a bottle (2018: 5.7-8.4 L/d; 2019: 3.8-5.6 L/d), as is common in the United States. Calves received ad libitum water and grain (Control: n = 20) with additional access to chopped mountaingrass hay (Hay: n = 29) from d 0. Intake was measured daily until the start of weaning at 49 to 51 d. Calves were ear-tagged at 3 ± 2 d (mean ± SD) of age with an accelerometer (SenseHub, Merck Animal Health) that classified rumination by minute continuously through 38 ± 10 d (mean ± SD) of age. We analyzed age of onset as the first of 3 consecutive days that a calf met the threshold level with a zero-inflated hurdle model. The logistic regression component of this model evaluated whether access to hay affected the likelihood of reaching each onset threshold. The second, conditional component of the model evaluated, among the calves that met a given threshold, if having access to hay affected the age at which they reached it. We found that calves consumed grain, hay, and water within 1 d of birth. Calves consumed small quantities of hay throughout the milk-fed period, typically <100 g/d, but this still led to differences in feeding development, with hay-fed calves reaching 5% rumination at younger ages, and a greater proportion reaching higher rumination thresholds compared with controls: specifically, 89% versus 53% reached the 15% threshold, and 61% versus 5% reached the 20% threshold (Hay vs. Control, respectively). Hay-fed calves also tended to be more likely to reach the 500-g grain threshold. There was no evidence that access to hay affected age at reaching higher rumination or grain thresholds, possibly because all calves were in similarly restrictive, milk-limited environments, where calories from solid food intake were particularly important. Calves with hay tended to reach the 3-L water threshold at a younger age than controls, but there was no evidence that hay affected likelihood of consuming increasing amounts of water. Overall, our results demonstrate that calves will consume grain, hay, and water as soon as they are provided, highlighting the importance of opportunities to perform species-specific feeding behaviors. Withholding hay from very young animals, which is common in the United States and allowable under the European Union 2008 directive, limits the development of motivated behaviors such as rumination.