ONA REPORT

published in

THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL


July 2016

Factors Impacting Shrink in Weaned Calves

by John Athington


For questions or comments regarding this publication contact: Dr. John Arthington, University of Florida, IFAS


In the beef industry, “shrink” is a term used to describe the change in animal body weight following a period of feed and/or water restriction. For weaned calves, shrink is typically associated with transportation, which further contributes to body weight loss. The November, 2004 Ona Report focused on this topic. Considering the importance of the topic, the approaching weaning season, and the presence of expanded information, now is a good time to revisit “calf shrink” and the role it plays in the well-being and overall value of Florida beef calves.

Shrink is a summation of weight change derived primarily from losses of feces and urine (gut fill) and tissue. As a proportion, studies have shown that about ½ of total body weight loss is associated with gut fill and the remainder coming from body tissues. Tissue losses are primarily the result of cellular fluid, sweating, and respiration losses. Environmental temperature impacts the proportion of these losses. As ambient temperature increases, the proportion of shrink associated with tissue loss also increases. In one study, steers transported in the heat of the summer lost 16% more body weight (vs. initial body weight) than similar steers shipped in the winter. This outcome presumably results in a greater degree of dehydration among calves weaned and transported during hot summer months.  In another study, calves shipped from their ranch of origin (approximately 560 miles) had nearly 30% greater shrink when transported in the summer vs. fall.  Of particular interest, the impact of season on calf shrink was only realized with ranch-derived calves. If calves were sale yard derived the magnitude of shrink was similar over all seasons evaluated.

Timing of gathering calves prior to load-out is another factor that impacts the shrink outcome.  This topic can be particularly impactful to Florida beef producers that are considering how they might gather, sort, and transport their calves in this year’s weaning season. In one study, the rate of shrink (% of body weight loss/hour) decreased rapidly as stocker steers (initial body weight = 680 lb) were held in pens following an early morning gathering.  In that study, steers shrank at a rate of 1.25% hourly in the first 2.5 hours, 0.61% hourly for the next 2.5 hours, and 0.16% hourly in the final 2 hours.  Clearly, the majority of shrink is occurring in the first 2 to 4 hours of fasting. At the time of weaning, Florida producers typically attempt to gather cow/calf pairs early in the morning. These early starts offer advantages relative to cooler morning temperatures and longer work days. Research from Kansas State revealed value in allowing calves to graze during morning hours prior to gathering.  In that study, calves gathered at 6 AM had less initial body weight and shrank at a faster pace compared to calves gathered at 9 AM when both groups were weighed at a 3 PM sale time.  This difference is attributed to the increased gut fill of calves that were allowed 3 extra hours of morning grazing prior to gathering. The rate of digestion and ruminal passage is impactful to this outcome.  Calves in this evaluation were heavier as gathering time was extended into the later morning, presumably due to a greater intake of grazed forage.

Dietary conditions impact shrink in grazing calves.  Calves consuming lush, more digestible forage will shrink more than calves consuming dry forage. This is due to differences in passage rate resulting in faster losses via feces and urine. For example, an Oklahoma State evaluation found that shrink can be reduced over 1% if calves grazing wheat pasture are maintained in drylot with dry hay and water access for 1 day prior to shipping. Preweaning and preconditioning calves prior to leaving the ranch has been purported as a management strategy to improve subsequent health and value to the buyer.  In terms of shrink, the effects of preconditioning have been variable.  Some studies have shown less shrink among preconditioned calves, while others have revealed more.  Variation is likely influenced by the time of day that non-preconditioned calves are gathered and separated from their dams, the digestibility of the diet provided to preconditioned calves prior to transport, and how long the calves stand in pens prior to transport.  These factors have the greatest influence on the magnitude of shrink among grazing calves. In a Florida study, freshly weaned calves had less shrink compared to 45-day weaned, preconditioned calves when both were exposed to a 24-hour trucking experience (7.2 vs. 9.6%).  The preconditioned calves in this study were provided approximately 10 lb/d of grain concentrate and free-choice access to ground grass hay.

Levels of stress resulting from cattle disposition and gathering and handling technique also impacts shrink. Dr. Ken Coffey, an expert on factors impacting cattle shrink, reported on experiences involving level of difficulty gathering cattle, whereas cattle that are difficult to gather tend to shrink more than those that move to the cowpens easily.  In Florida, a logical consideration to reduce gathering stress and subsequent shrink is to gradually move herds toward the pens over a couple weeks prior to weaning and transport.  In doing so, producers should allow pastures adjacent to cowpens to accumulate forage in the month prior to weaning. This will ensure adequate forage for grazing when the herd arrives prior to weaning.    

During summer weaning in Florida, many producers understand the importance of getting cattle “moving down the road” after weaning and loading.  Truck movement provides air circulation over the cattle and helps dissipate increased body heat that has been generated during weaning and sorting.  Data from a recent study at the Range Cattle REC (Ona) revealed peak vaginal temperatures among August-transported heifer calves immediately after sorting and loading with rapid reductions following truck movement (Figure 1).   In addition, the experience of the truck driver also appears to influence calf outcomes.  In a survey of Canadian cattle truck drivers, cattle driven by drivers with 6 or more years of experience had less body weight shrink and less likelihood of ambulatory problems compared to drivers with fewer years of experience.
 
Many factors influence shrink in beef calves.  The traditional summer-weaning in Florida presents increased pressure on the physiological responses impacting shrink.  Special consideration to length of fasting, time of gathering, air temperature, and trucking conditions can all impact body weight losses due to shrink. Ultimately, these factors impact both calf well-being and economic value.


or7-16fig1
Figure 1.  Vaginal temperature of weaned heifers transported 22 hours in south Florida in August. Temperature peaked shortly after loading and declined thereafter while truck was moving.

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You are invited to join me by webinar on July 12, at noon, when I will present the information covered in this Ona Report. Afterwards, I will be available to answer questions. To participate in this webinar you simply need to register online: https://attendee.gotowebinar.com/register/154919364366849793 or visit our website: https://rcrec-ona.ifas., a link will be available there. There is no cost to attend.


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