Bunk Management Affects Feeding Behaviour and Intake

Regulated vs Ad Libitum feeding
"Regulated feed delivery" is another term for what is commonly known in the feedlot industry as "slick-bunk management". This style of bunk management concerns some feedlot managers since they assume that dominant animals will over eat (resulting in higher morbidity and mortality rates) and that reducing feed availability means reduced intake. The goal of slick-bunk feed delivery is to improve performance by regulating the average pen intake to reduce digestive problems resulting from over consumption of feed.

Cattle feeders have traditionally tried to maximize animal intake by ensuring continual availability of feed. This strategy is known as ad libitum or free choice feeding. Bunk management strategies in this scenario focus primarily on maintaining bunk hygiene by reducing left over feed to a small percentage of what was delivered. Attempts to target intake to a level at which weigh backs are reduced to crumbs often amplifies fluctuations in intake as feed callers overcompensate in their attempts to achieve the balance between too much and too little. Slick-bunk managers generally acknowledge that intake could be temporarily increased by 0.5 or 1 kg per head if more feed was delivered. However, the extra feed is typically thought to cause mild acidosis, which would result in a subsequent decline of 1 or 2 kg of intake/day. These managers believe that higher mean intakes can be maintained by avoiding intake crashes than by achieving the peaks. Whereas ad libitum feeding attempts to maximize intake on a daily basis, slick-bunk feeding strives to maximize mean intake over the course of the feeding period.

Because there are frequently no weigh backs in a slick-bunk feeding strategy, bunk managers must rely heavily on interpreting cattle behavior (interest and feeding aggression) when making feed calls. Subtle changes in cattle interest are considered along with bunk condition, when making the feed call. It is conceivable that when feed calls are made primarily on the number of animals displaying interest rather than on the quantity (or lack) of weigh backs remaining, feed delivery more accurately reflects average appetite, and average intake for the pen may be more representative of individual intakes.

It is believed that slick-bunk management typically results in animals becoming meal eaters and daily variation in total feed intake is reduced by feed restriction. Surprisingly, in the one reported study in which ruminal pH was actually measured in steers fed either to meet ad libitum intake or using a slick-bunk feed delivery strategy, the cattle fed with a slick-bunk protocol had lower and more variable ruminal pH, as a consequence of changes in their eating patterns (Fanning et al., 1999). Larger meals and a faster rate of eating resulted in a greater pH decline in the slick-bunk fed cattle. Those findings are in contrast with a study at the Lethbridge Research Centre that reported individual cattle exhibited reduced day-to-day variation in time spent at the bunk when they were limit-fed (to 95% of ad libitum intake) than when they were on full feed.

The assumption that animal responses to bunk management (if present) are a direct result of reduced acidosis is drawn from pure speculation. The observations reported by Fanning and his coworkers contradict these assumptions, but this does not necessarily discredit the practice of slick-bunk management to improve performance. Increased eating rates and less frequent meals are commonly observed among cattle with limited access to feed. Increased eating rates have been associated with improved performance in sheep and with increased intake and performance by cattle. Some researchers have identified an apparent negative correlation between time spent at the feed bunk and average daily gain, which suggests that the cattle with the best growth performance were likely eating faster. However, in a recent study completed at the Lethbridge Research Centre indicated that, during the backgrounding phase, cattle that spent the most time at the bunk had the highest ADGs. This means that data of this type must be interpreted carefully since the effects can be drastically different depending on how and when the animals are being feed. At this time the mechanism(s) by which increased eating rates might influence performance are unknown.

Number of feed deliveries
It is generally recognized that cattle come to the feedbunk when new feed deliveries are made. This is presumably the benefit of feeding cattle more than once a day. Research indicates that the highest percentage of cattle feeding coincide with feed deliveries. We can interpret this to mean that feed trucks stimulate feeding and that increasing the number of deliveries should increase the number of meals and therefore performance. However, on looking closer the same study showed that less than 50% of the cattle were eating. This means that cattle were not hungry or not stimulated by the presence of the truck. Other studies have shown that lactating and dry cows given an average of 7 kg of concentrates per day did not change their silage intake or feeding pattern irrespective of whether the concentrate was given 2 or 22 feeds per day. These results can be partly explained by the fact that cattle are crepuscular (having an innate diurnal pattern of activity that is highest at dawn and dusk). Stricklin (1986) indicated that eating episodes were more synchronous with sunset than feed delivery. Peak eating patterns by ad libitum fed cattle are more influenced by the time of day rather than feed delivery time (similar to free range cattle).

Time of feed deliveries
It has been shown that most of the dry matter intake by cattle is consumed in late afternoon and evening hours. This period corresponds with lowest environmental heat load during the summer and coldest conditions in the winter. There may be some advantage to feeding cattle in the evening rather than in the morning. During the summer, environmental heat can stress cattle. The heat of fermentation and the heat increment associated with eating can contribute to the heat load and increase the animals maintenance energy requirement. During the winter the lowest temperatures occur at night and heat generated by fermentation contributes to the maintenance of core body temperature. These facts suggest that there would be some overall energetic efficiency if feed was consumed at night in either the summer or the winter.

A study conducted by Pritchard and Knutsen (1995) compared feeding cattle (ad libitum) once at either 0700 or 1600 or twice at 0700 and 1600. They found that cattle fed once daily in the evening had higher ADGs and feed efficiencies than cattle fed in the morning. Dry matter intake was not affected by time of feeding. Performance of cattle fed twice a day was intermediate between AM and PM treatments. Once daily feeding caused more feed to be consumed in the evening hours. Similar findings were have been reported for limit fed cattle. Feeding was done once daily at either 0800 or 2000. Evening feeding increased ADG by 18% despite a fixed and equal caloric intake. These studies indicate that eating behavior may be manipulated to enhance performance by altering the feed delivery schedule. The best feed delivery programs for various types of cattle during various stages between backgrounding and finishing have yet to be determined.

Consistency of amount of feed delivered
It is commonly assumed that fluctuations in intake can cause acidosis and animals to go off feed. This belief, held by most feedlot producers, stems from research conducted by Galyean et al. (1992) indicating that cattle whose feeding level fluctuated (10%) daily had 6% poorer gains and 7% poorer feed efficiencies than those fed at a constant level. The conclusion drawn from this research is that the impaired performance was a result of the acidosis arising from intake variation, even though ruminal pH was not measured. This theory, generated from a single trial, remains prevalent despite a mounting body of research that suggests otherwise. A study conducted by Cooper et al. (1998a; 1998b) monitored ruminal pH in a metabolism trial, and animal performance in a finishing trial which included deliberate fluctuations in intake. Ruminal pH was lower for limit-fed cattle when day-to-day intakes were varied by 1.4 kg, but there were no differences in pH when the cattle were fed to meet ad libitum intake. During the study, an equipment malfunction delayed feeding for 4 h. At that point it was observed that delayed feeding (such as could arise in a commercial lot, with equipment breakdown or inconsistent timing of delivery) can have a greater effect on ruminal pH than fluctuating quantities of feed. In the finishing trial varying the amount fed by 1.8 kg per day numerically increased intake but did not affect performance.

It is feasible that cattle become adapted to consistent variations in the quantity of feed delivered, thereby decreasing the negative effects of feed intake fluctuation. Observations of the comfort and eating behaviors of milk-fed dairy calves indicate that predictable inconsistencies are tolerated better than are random inconsistencies.

Dr. Karen Schwartzkopf-Genswein
Agriculture and Agri-Food Canada, Research Centre, Lethbridge

Darryl Gibb
Agriculture and Agri-Food Canada, Research Centre, Lethbridge

Southern Alberta Beef Review - January, 2000. Volume 2, Issue 1