Nutritional and lactational responses of Murciano-Granadina dairy goats to heat stress

The aim of this thesis was to study the effects of heat stress on the nutritional and lactational performances of dairy goats. Eight multiparous Murciano-Granadina dairy goats (194 plus minus 3 DIM; 1.53 plus minus 0.04 L per d; 43.5 plus minus 2.6 kg BW) were maintained in metabolic cages under 2 different controlled conditions of temperature and relative humidity (THI, temperature humidity index): thermal neutral (TN; 15 to 20 degrees C and 40 per cent; THI, 59 to 65) and heat stress (HS; 12 h day at 37 degrees C and 40 per cent; THI equal to 85; and 12 h night at 30.5 degrees C and 40 per cent humidity; THI equal to 77). The experimental design was a crossover with 2 periods, 35 d and 4 goats each. Both goat groups were fed a dry diet consisting of ad libitum dehydrated fescue, 0.65 kg alfalfa pellets, and 0.8 kg concentrate. Rectal temperature, respiration rate, feed intake, water consumption, and milk yield were recorded daily. Milk and blood samples were collected weekly for analysis of milk components, and NEFA and haptoglobin in the blood. At d 25 of each period, additional blood samples were taken for analysis of metabolites and indicators of the acid-base balance. Feces and urine were collected during the last 5 d of each period (d 31 to 35) for digestibility, N balance and fecal corticosterone. Rectal temperature (38.66 vs. 39.24 degrees C), respiration rate (34 vs. 82 breaths per min), water consumption (5.5 vs. 11.1 L per d), and body water evaporation (1.1 vs. 3.3 L per d) increased, while feed intake (2.0 vs. 1.6 kg per d) decreased for TN and HS goats, respectively. Feed intake under HS conditions gradually decreased until d 7 (-40 per cent), partially recovered from d 7 until d 19 (-31 per cent), and remained constant (-14 per cent) thereafter (d 19 to 35), showing an adaptation of goats to HS. Blood NEFA and haptoglobin had greatest values in HS goats at d 7. Despite the reduced feed intake, milk yield (1.23 L per d) and FCM (1.37 L per d) did not vary by treatments, but milk of HS goats contained lower protein (3.36 vs. 3.84 per cent) and casein (2.84 vs. 3.21 per cent) than TN goats. Panting reduced concentration and pressure of CO2 in blood of HS goats, but they were able to maintain their blood pH similar to TN group by lowering bicarbonate and increasing Cl- blood concentrations. There was a tendency for greater digestibility in HS goats, which may partially compensate for the reduction in feed intake. In conclusion, goats were able to partially recover their feed intake after wk 3 of heat stress resulting in a similar milk yield but with lower milk protein content. Further studies are necessary to evaluate the response of early lactating dairy goats to heat stress conditions.