Reducing rumen starch fermentation of wheat with three percent sodium hydroxide has the potential to ameliorate the effect of heat stress in grain-fed wethers

Abstract

Selection for high productivity has resulted in ruminants adversely affected by heat stress (HS) due to their high metabolic rate and feed intake. One mechanism to ameliorate HS is to reduce the forage-to-concentrate ratio in the diet, although the inclusion of readily fermentable grains can reduce heat tolerance. The aim of these experiments was to investigate a chemical method for reducing the rate of fermentation of wheat and its effect on improving heat tolerance in sheep. In the first experiment, fermentation kinetics and buffered rumen fluid pH variation during in vitro incubation of corn, wheat, and 3% NaOH-treated wheat grains were compared. This experiment showed that corn and 3% NaOH-treated wheat had a slower (-23 and -22%, respectively; < 0.001) rate of gas production and elevated buffered rumen fluid pH ( < 0.001) compared with wheat. In the second experiment, 31 Merino × Poll Dorset wethers were housed in 2 climate-controlled rooms and were fed either corn grain plus forage (42.7% starch; were fed either corn grain plus forage (CD), wheat grain plus forage (WD) or 3 % NaOH-treated wheat plus forage (TWD) during 3 experimental periods: period 1 (P1), which consisted of 7 d of thermoneutral conditions (18 to 21°C and 40 to 50% relative humidity) and 1.7 times maintenance feed intake; period 2 (P2), which consisted of 7 d of HS (28 to 38°C and 30 to 50% relative humidity) and the same feed intake as in P1; and period 3 (P3), which consisted of 7 d of HS as in P2 and 2 times maintenance feed intake in a randomized control experiment. Water was offered ad libitum. The impact of HS was quantified by increases in rectal temperature, respiration rate (RR), and flank skin temperature (FT); variations in blood acid-base balance; and glucose, NEFA, and heat shock protein 70 (HSP-70) plasma concentrations. All physiological variables were elevated during HS, especially when wethers had greater feed intake (P3). Wethers fed CD had lower RR, rectal temperature, and FT than wethers fed WD ( < 0.001) and wethers fed TWD had lower RR and FT than wethers fed WD during HS ( < 0.05). There were reductions in blood CO, HCO3, and base excess concurrent with increases in blood partial pressure of O and pH during HS ( < 0.05). Heat stress reduced plasma NEFA and glucose concentrations whereas it increased prolactin ( < 0.05). Prolactin and HSP-70 plasma concentrations were greater for WD-fed wethers ( < 0.001) associated with Prolactin and HSP-70 plasma concentrations were greater for WD fed wethers (P < 0.001) during HS. These data indicate that the slow rate of rumen fermentation of CD and TWD can reduce the heat released during feed fermentation in the rumen, improving heat tolerance in sheep.

Figure 1.

Gas production curves of incubation of ground corn, untreated wheat, and 3% NaOH–treated wheat grains (NaOH-wheat). Gas production curves are representative of 24 h of incubation at 39°C in buffered rumen fluid. Curves are the result of the application of the Gompertz model to the estimated means of kinetic parameter of 28 replicates per grain after curve fitting: y = A + C^{exp{−exp[−B(X−M)]}}, in which B is the rate of gas production (mL/h), M is the time (h) at which the maximum (max) rate of gas production is reached, C is the maximum gas produced (maximum gas mL/g DM), A is the y-intercept, and X is the time (h) of incubation (P < 0.001).

Figure 2.

Buffered rumen fluid pH at 5, 8, and 24 h of in vitro incubation of ground corn, untreated wheat, and 3% NaOH–treated wheat grains (NaOH-wheat). Results are estimated means and pooled SED (0.057) for the grain × incubation time effect (P < 0.001).

Figure 3.

Relationship between respiration rate (a) and rectal temperature (b) and time of day and experimental period: period 1 (P1; thermoneutral [18 to 21°C and 40 to 50% relative humidity] and restricted feeding [1.7 × maintenance{M}]), period 2 (P2; heat stress [28 to 38°C and 30 to 50% relative humidity and restricted feeding [1.7 × M]), and period 3 (P3; heat stress and unrestricted feeding [2 × M]) in wethers fed corn, wheat, or a 3% NaOH–treated wheat–based diet (NaOH-wheat). Results are means pooled across days and pooled SED for the diet × time × period interaction. The P-values for the effects of diet, period, time, diet × period, diet × time, period × time, and diet × period × time were <0.001, <0.001, <0.001, <0.001, 0.031, <0.001, and 0.48, respectively, for respiration rate and <0.001, <0.001, <0.001, 0.006, 0.005, <0.001, and 0.59, respectively, for rectal temperature. Max Heat = maximum room temperature between 0900 and 1700 h.

Figure 4.

Relationship between left flank skin temperature and experimental period: period 1 (P1; thermoneutral [18 to 21°C and 40 to 50% relative humidity] and restricted feeding [1.7 × maintenance{M}]), period 2 (P2; heat stress [28 to 38°C and 30 to 50% relative humidity and restricted feeding [1.7 × M]), and period 3 (P3; heat stress and unrestricted feeding [2 × M]) in wethers fed corn, wheat, or 3% NaOH–treated wheat–based diet (NaOH-wheat). Results are means pooled across days and pooled SED for the diet × period interaction. The P-values for the effects of diet, period, and diet × period were <0.001, <0.001, and <0.001, respectively.

Figure 5.

Relationship between heart rate and time of day and experimental period: period 1 (P1; thermoneutral [18 to 21°C and 40 to 50% relative humidity] and restricted feeding [1.7 × maintenance{M}]), period 2 (P2; heat stress [28 to 38°C and 30 to 50% relative humidity and restricted feeding [1.7 × M]), and period 3 (P3; heat stress and unrestricted feeding [2 × M]) in wethers fed corn, wheat, or 3% NaOH–treated wheat–based diet (NaOH-wheat). Results are means pooled across days and pooled SED for the diet × time × period interaction for heart rate. The P-values for the effects of diet, period, time, diet × period, diet × time, period × time, and diet × period × time were <0.001, 0.005, <0.001, 0.349, 0.214, 0.236, and 0.92, respectively. Max Heat = maximum room temperature between 0900 and 1700 h.