When a calf swallows, solid food such as starter grain moves down the esophagus and passes through an opening called the esophageal groove just before it enters the rumen. Prior to weaning, milk and milk replacer take a different route. Factors such as suckling, anticipation, and a variety of sensual and neural stimuli cause muscles around the esophageal groove to contract. These muscular contractions close the groove, allowing milk and milk replacer to bypass the rumen and flow directly into the abomasum for digestion. See Figure 1.
The presence of milk in the rumen and reticulum is considered to be abnormal and is undesirable from a physiological and nutritional standpoint. Wise and Anderson (1939) observed that when calves suckled milk through a nipple, the frequency of exceeded 3 milliliters. When milk was drunk from an open pail, the frequency of milk entering the rumen was over 40%. The amount that entered the rumen varied from a few milliliters to over 50% of the milk consumed.
Later research by Abe et al (1978) concluded that the efficiency of esophageal groove closure was similar for both nipple and open pail feeding, with very little milk replacer entering the rumen. Hegland et al (1957) found that stimuli such as the presence and activity of farm workers who fed the calves sometimes caused complete closure of the esophageal groove in certain calves.
How do we reconcile what may appear to be different research findings? Each study demonstrates that efficient closure of the esophageal groove is possible with either feeding method. Wise and Anderson (1942) found that the greatest portion of milk entering the rumen was during the first few seconds of drinking, whether calves drank from an open pail or suckled a nipple. The reasons for milk escaping the esophageal groove can be due to a number of factors including variation in the stimuli for groove closure and the larger swallow sizes associated with drinking from an open pail compared to suckling a nipple. In the final analysis, feeding through a nipple provided consistent, efficient groove closure. Feeding with an open pail, however, appears to have a greater potential for variability in animal response.
Ruminal drinking. Feeding in open pails can occasionally lead to ¡°ruminal drinking¡±, a situation where most of the milk or milk replacer consumed enters the rumen. Calves that are ruminal drinkers can develop feed intake and weight gain problems ¨C a condition most commonly observed in veal calves. Under normal conditions, only about 3% of the milk or milk replacer consumed by veal calves enters the rumen (Davis and Drackley, 1998). However, when large volumes of milk repeatedly enter the rumen, calves can develop an overall appearance commonly referred to as ¡°hay belly¡±. Although the term is incorrect in this case, calves can develop the classic short, squatty stature with a low-slung distended belly. The situation can lead to serious metabolic problems resulting in low rumen pH, growth of yeast rather than normal rumen microbes, small distorted rumen papillae, poor VFA production and low cellulose digestion. Ruminal drinkers are typically very aggressive drinkers and corrective action involves slowing down the drinking process. Providing multiple smaller feedings, using a floating nipple in the bucket or feeding with a bottle and nipple or a nipple pail are techniques that slow the...