Digestibility and Volatile Fatty Acids in Steers and Wethers at 21 and 32 C Ambient Temperature 1 H. LIPPKE Texas Agricultural Experiment Station
Angleton 77515
increase being due entirely to a more efficient fiber digestion (16). The total of these effects is approximately four percentage units in moderately stressed cows (3, 4). Whether there is a direct influence of heat stress on digestibility is not clear. When feed intake is controlled, a trend toward higher digestibility with heat stress is often evident, but increases have been statistically significant in only a few trials (1, 6). In previous trials with wethers (unpublished data), heat stress was associated with changes in ruminal volatile fatty acid (VFA) ratios favoring acetic acid. Such a change might increase the heat load on an animal already stressed. Since experimental animals used in forage quality studies at this station are frequently subject to at least mild heat stress, a study was initiated to clarify the effects of heat stress on digestion. This report presents the results of digestion trials and ruminal VFA determinations for steers and wethers maintained at 21 or 32 C.
ABSTRACT
Four steers and four closely shorn wethers, each with ruminal fistula, were used in a reversal experiment to compare the direct effect of 21 and 32 C ambient temperatures (70% relative humidity) on dry matter digestibility and ruminal volatile fatty acid concentrations. Pelleted alfalfa intake was held constant at 2.3% of body weight. Orts were placed into the rumen via fistula. Dry matter digestibility was determined by total collection during the last 7 days of each trial. On one of the last 2 days, ruminal fluid was sampled 11 times within 13 h. At 32 C, the steers suffered considerable heat stress as evidenced by a mean 1.6 C increase in body temperature, reduced heart rate, and greatly increased respiration rate. The wethers, however, exhibited only increased respiration rate at 32 C. Extent of digestion increased five percentage units for the steers but did not change for the wethers at the high temperature. Temperature caused only slight shifts in ruminal volatile fatty acid concentrations. Volatile fatty acid concentrations were much higher in steers than in wethers. The frequently observed effects of heat stress on intake and digestibility are mediated by changes in rate of passage as a result of altered thyroid secretion rates.
EXPERIMENTAL PROCEDURE Animals
INTRODUCTION
The drastic depression in feed intake by cattle under serious heat stress was established early in the Missouri Station's study of environmental physiology (13). Similar observations have been reported for sheep (15). Reduced intake, in turn, increases digestibility (2), the
Received March 7, 1975.
1Technical paper TA 11693 of the Texas Agricultural Experiment Station, College Station.
Responses of four steers, two Holstein and to Jersey, ranging from 321 to 382 kg body weight (BW) and four closely shorn, mature wethers weighing 34.0 to 47.5 kg were compared in a reversal experiment wherein the animals were housed at 21 or 32 C. All animals were ruminally fistulated to allow forced feeding when necessary. During preliminary periods, animals were allowed to move about freely in open lots and establish voluntary intakes of alfalfa pellets (40% neutral detergent fiber, 21% crude protein). Daily feed intake was restricted to 2.3% of BW when animals were placed in temperature controlled chambers at the beginning of each trial period. This amount of feeding assured voluntary consumption of the entire ration on days i to 4 when both chambers were
1860
HEAT STRESS AND RUMINANT DIGESTION maintained at 21 C. On day 5, the temperature of chamber 2 was increased to 32 C, and fecal collection harnesses were placed on the steers. Total fecal collection was on days 7 to 13. One steer of each breed and two wethers were assigned randomly to the two chambers at the beginning of the adjustment period of trial 1. They were placed in opposite chambers for trial 2. All animals were fed .5 ration at 0800 and 1630 h. Generally, at each feeding, orts from the previous feeding were placed into the rumen. Water and mineral blocks were available at all times. Facilities
Two conventional metabolism cages for sheep and two cattle units, each consisting of feed trough, automatic water cup, swivelmounted stanchion, and heavy rubber mat, were placed in each of two chambers (8.76 m x 2.72 m × 2.49 m high) equipped for heating or cooling. A hygrothermograph was placed in each chamber level with the heads of both cattle and sheep. The equipment in chamber 2 was modified to keep relative humidity (RH) at 70% with the 32 C temperature. During trial 1, the heating rate in chamber 2 exceeded the capacity of humidifying equipment causing a downward spike in RH when the furnace was actuated. A rapid recovery to 60% RH followed and then a slower rise to 68%. Heating rate also caused temperature to overshoot the thermostat setting producing a 4 ° oscillation about the desired 32 C. For trial 2, heating rate of the furnace was reduced, bringing temperature and humidity fluctuations within the relatively narrow sensitivity of the controlling units. Relative humidity in chamber 1 (21 C) ranged from 60 to 90% during the first trial. Equipment modifications made between trials reduced oscillations in humidity to 6 percentage units about the 70% mean value. Digestibility
Digestible dry matter (DDM) was determined in 7-day trials using conventional total collection procedure~. Rations for an entire trial were weighed and sampled at the beginning of each adjustment period. Feces were removed from the chambers four times daily to reduce
1861
artifacts due to continued fermentation of feces after being voided. Feces were composited daily. All feces from wethers and 10% of the wet feces from each steer were dried at 65 C, as were feed samples. Neutral detergent fiber (NDF) and acid detergent fiber (ADF) of feed and feces were determined according to standard procedures (5). Ruminal V F A
On each of the final 2 days of each trial, ruminal fluid samples were taken at - . 1 , .5, .75, 1.0, 1.5, 2, 3, 4, 6, 9, and 13 h after the morning feeding. For those animals being sampied, orts from the previous feeding were placed into the rumen at - 8 h. Feed not eaten within .75 h also was placed into the rumen via fistula. Time required for sample processing allowed only one wether and one steer from each chamber to be sampled daily. Twentyfive-milliliter samples were aspirated through a strainer (14) located in the ventral sac of the rumen, pH was determined immediately. Each sample was then mixed with 5 ml of freshly prepared 25% metaphosphoric acid, allowed to stand for 30 rain, and centrifuged at 18,000 x g for 20 min. Supernatant was decanted into bottles, sealed, and frozen for later analysis. Volatile fatty acid determinations were b y gas-liquid chromatography with a 3.2 m m x 150-cm teflon column packed with polyethylene glycol 400 monostearate and phosphoric acid adsorbed onto .24/.18 m m Chromosorb P-AW. Alpha-ethyl-N-butyric acid was an internal standard. Total acids were determined by steam distillation and titration. Physiological Measurements
In trial 1, rectal temperatures of all animals were measured with standard veterinary thermometers on days - 1 (four times), 3, 5, 7, and 9 (three times) after differential chamber temperatures were established. In trial 2, these measurements were on days 0 (three times), 2, 4, 5, 6, and 8. Heart and respiration rates were measured with a stethoscope and visually on the last day of both trials and just before establishment of differential temperatures in trial 2. Journal of Dairy Science Vol. 58, No. 12
1862
LIPPKE
TABLE 1. Analysis of variance for ruminal fluid pH and VFA data. Source of variation
df
Trial (Tr) Day/ttials (D/Tr) Temperature (Te) Species (S) Te×S Tr X D × Te × S [error (a)] Time (Ti) Te X Ti S × Ti Te X S × Ti Error (b)
1 2 1 1 1 9 10 10 10 10 120
Statistical Analyses
Digestibility a n d physiological d a t a were s u b j e c t e d t o analysis of variance for r a n d o m ized c o m p l e t e b l o c k design. R u m i n a l V F A a n d pH data were t r e a t e d as a m o d i f i e d split-plot design ( T a b l e 1). RESULTS
In trial 1, 30% o f t h e r a t i o n o f o n e steer a t 32 C was r e f u s e d a n d h a d t o be p l a c e d i n t o t h e r u m e n . In trial 2, these values were 4 0 % a n d 60% for t h e t w o steers at 32 C. T h e s e s a m e steers r e j e c t e d negligible a m o u n t s at t h e c o o l e r t e m p e r a t u r e . No orts were e n c o u n t e r e d a m o n g t h e wethers. O f t h e p a r a m e t e r s in T a b l e 2, o n l y respirat i o n rate failed to d e m o n s t r a t e a s i g n i f i c a n t a m b i e n t t e m p e r a t u r e x species i n t e r a c t i o n .
C o n s e q u e n t l y , tests for t r e a t m e n t d i f f e r e n c e s were w i t h i n species. Rectal temperature measurements during the a d j u s t m e n t periods were w i t h i n t h e n o r m a l range. B o d y t e m p e r a t u r e of t h e steers appare n t l y increased w i t h i n 2 days a f t e r t h e steers were s u b j e c t e d t o t h e 32 C a m b i e n t t e m p e r a ture. Since n o changes in rectal t e m p e r a t u r e were d e p e n d e n t o n time, all m e a s u r e m e n t s d u r i n g t h e trials are i n c l u d e d in t h e m e a n values in T a b l e 2. While t h e r e is little q u e s t i o n t h a t t h e steers s u f f e r e d c o n s i d e r a b l e h e a t stress, t h e w e t h e r s were a p p a r e n t l y able t o c o n t r o l b o d y t e m p e r a t u r e b y increasing r e s p i r a t i o n rate. T h e differential r e s p o n s e in h e a r t rates, resulting in an a m b i e n t t e m p e r a t u r e × species i n t e r a c t i o n , c o n f i r m s t h e lack of a s i g n i f i c a n t heat-stress c o n d i t i o n in t h e w e t h e r s . T h e effect o f high a m b i e n t t e m p e r a t u r e a n d t h e r e s u l t a n t h e a t stress o n D D M a p p r o a c h e d significance ( P < . 0 5 ) for t h e steers. D e t e r m i n a t i o n s of DDM w i t h h e a t - s t r e s s e d steers were marked by considerable variation (coefficient of v a r i a t i o n = 5.4%). T h e c o e f f i c i e n t s o f variat i o n for digestibility o f N D F a n d A D F in steers were 11.1% a n d 11.7%. Thus, t h e s e e m i n g l y large m e a n differences ( T a b l e 2) were n o t significant ( P > . I ) . Detailed analyses o f c o n c e n t r a t i o n p a t t e r n s of r u m i n a l V F A will b e in a s u b s e q u e n t paper. Because s a m p l i n g t i m e s f o r V F A a n d pH were n o t s p a c e d u n i f o r m l y , each value was w e i g h t e d p r o p o r t i o n a l l y t o derive t h e m e a n s in T a b l e 3. T h e t h r e e m a j o r acids were h i g h e r ( P < . 0 5 ) for steers t h a n for wethers. Differences in valeric
TABLE 2. Digestion coefficients, rectal temperature, respiration rate, and heart rate for steers and wethers maintained at 21 or 32 C ambient temperature. Steers
Wethers
Item
21
32
21
32
Digestibility, % 1 Dry matter Neutral detergent fiber Acid detergent fiber a Neutral detergent solubles 2 Rectal temperature (C) 2 Respiration rate (min-1 ) 2 Heart rate (min-1 )
62.2 a 47.9 44.2 71.8c 38.7 c 26 c 82 c
67.5 b 54.3 51.1 75.8 d 40.3 d 115 d 69 d
62.7 50.2 45.8 70.6 39.1 76 c 78
62.8 49.3 46.6 71.3 39.2 154 d 84
1 Different letter superscripts indicate differences (P