The control mechanisms of milk production in crossbred goat under tropical condition : effects of high dietary cation and anion difference

Abstract

The present study was carried out to evaluate the effect of high dietary cation and anion difference (DCAD) rations in dairy goats fed under high ambient temperature (HTa) on milk production in relation to physiological responses, including diurnal variations in eating and meal patterns, ruminal function and nutrition digestibility, water balance and body fluid compartments. Ten crossbred dairy goats during peri-parturition period were selected and divided into two groups of five animals each. Experimental diets were control DCAD (control, 22.8 mEq/100 g DM) and high DCAD (DCAD, 39.1 mEq/100 g DM). The composition of two diets consisted of 44% corn stover by-product silage and 56% concentrate. During the experimental period from the 2nd to 8th weeks of postpartum (PP-2 and PP-8), goats were fed twice daily either with the control or DCAD total mix ration with free access to water. The environmental conditions, rectal temperature (Tr) and respiratory rate (RR) in the present experiment indicated that goats were fed under HTa conditions (average peak THI = 85.2) and were in the stage of heat stress. The percentage change of Tr from DCAD group was lower than control group between 09:00 h and 13:00 h. High DCAD apparently increased eating and meal patterns compared with the control. Dry matter intake/body weight (DMI/BW) tended to increase throughout experiment and significantly higher than in animals fed with high DCAD at PP-8 (P<0.05), but milk yield and composition were similar between groups. An increase in DMI from DCAD group at PP-8 mainly came from increase in meal size and duration in accordance with the improvement of ruminal function and nutrition digestibility. However, the plasma leptin concentration from DCAD was higher than those from control. The concentrations of plasma K+, Cl- and osmolality was not affected by DCAD at 09:00 h and 16:00 h, but plasma Na+ level and cation and anion difference (CAD) from DCAD increased at 16:00 h. Goats in DCAD group drank more water than control. However urine volume and plasma ADH concentration were not different between groups. As a result, apparent water balance was higher from DCAD group during 24 h. There were no effects of DCAD on plasma and blood volumes, but tended to increase in extracellular fluid and thereby increasing total body water. The results from current study indicate that dairy goats fed with high DCAD tended to increase DMI/BW by increasing meal size, meal duration. An increase in DMI/BW apparently came from an improving gastrointestinal tract function and was independent from the action of leptin. In addition, high DCAD resulted greater in total body water and apparent water balance. These results have contributed the process of adaptation for evaporative cooling and would be useful in slowing down the elevation in Tr under HTa.