Effects of High Ambient Temperature and Bypass Protein Supplementation on Performance of Crossbred Lambs

Abstract

The objective of this experiment was to study the effect of dietary supplementation with a source of
bypass protein (cottonseed meal, CSM) on the performance of growing crossbred wethers fed low
digestibility roughage at high ambient temperature. Sixteen unshorn Border Leicester x Merino lambs
aged 6 months and weighing 24.90.37 kg were allocated into a 2x2 factorial design (2 diets and 2
ambient temperatures). Each group was fed either wheaten chaff + 2% urea ad libitum or wheaten chaff +
1% urea ad libitum + 100g/d cottonseed meal. The results show that respiration rate and rectal temperature
were significantly affected by ambient temperature (P<0.01), but not by diet. Total dry matter intake was
significantly higher (P<0.01) at lower temperature than at high temperature, and significantly higher
(P=0.04) on the supplemented diet than on the control diet. Water intake tended (P=0.06) to be higher on
the CSM+ diet than on the control diet, and was significantly at greater (P<0.01) high ambient temperature
than at low temperature. The concentration of NH3-N in the rumen fluid (ranged from 310±17 to 413±20
mg N/L) did not differ significantly either between diets or ambient temperatures. Blood plasma urea
nitrogen concentration was significantly higher in sheep on the control diet than in those on the
supplemented diet, and at low than high temperature. The organic matter digestibility was not significantly
affected by temperature and diet. The rate of live weight gain was significantly (P<0.01) reduced by high
ambient temperature and the interaction between diet and temperature was significant (P<0.01). At low
temperature, lambs on the supplemented diet grew significantly faster than those on the control diet, but
not at high temperature. In conclusion, high ambient temperature increases respiration rate, rectal
temperature and water intake but reduces dry matter intake and live weight gain. Supplementation with
100 g/d of cottonseed meal could not outweigh the depressing effects of constant high ambient
temperature and humidity imposed in this experiment.