4.1. Methodological issues
The RSP method was chosen because it effectively narrows down the dose window to pinpoint an exact dosage [17] and [20]. The method permits the dose level to be adjusted based on the amount of milk in the rumen, allowing us to determine the volume at which the abomasal capacity would be exceeded. Between- and within-patient RSP designs are generally analysed with isotonic regression analysis [17] and [20]. However, no meal size in the current study turned out to cause milk to enter the rumen. This means that there were no quantiles to estimate and hence isotonic regression could not be applied to determine an optimal dose level based on abomasal capacity.
Milk is easily digestible and abomasal emptying in young calves normally starts within a few minutes [21]. Radiographs were taken during and immediately after the milk intake when backflow is most likely to occur. Ruminal emptying can take up to 48 h [3]. To avoid confusion from barium sulphate potentially remaining in the forestomachs, test sessions were performed with 48 h intervals.
Teat feeding, and the use of a small aperture teat, are widely recommended in the literature [22], [23], [24] and [25], and was also the method of choice in this experiment. Compared to non-sucking methods, teat feeding has advantages like stimulating the oesophageal reflex, and causing fewer sequential openings and closings of the oesophageal groove [26].