Variações anátomo-fisiológicas face à aclimatação sazonal - estudo em vacas leiteiras com diferente potencial leiteiro

O presente estudo teve como objetivo a caracterização de variações anátomo-fisiológicas que decorrem do processo de aclimatação sazonal em bovinos leiteiros com alto (Altas) e baixo (Baixas) potencial leiteiro, com vista a selecionar animais que conciliem bons desempenhos produtivos e índices de adaptabilidade que possibilitem a mudança de homeostase face às condições climáticas presentes no clima mediterrânico. O estudo foi realizado numa herdade comercial, situada no Alentejo, utilizando 13 vacas multíparas (6 Baixas e 7 Altas), durante três períodos: P1 (animais aclimatados ao verão; presença de stresse térmico); P2 (animais aclimatados ao verão; termoneutralidade); P3 (animais aclimatados ao inverno; termoneutralidade). Em stresse térmico (P1), verificaram-se maiores esforços termolíticos e maiores armazenamentos de calor no grupo das Altas. Observou-se também que a produção de leite das Altas foi afetada pelo stresse térmico, evidênciando uma redução 24-48h após os valores de temperatura retal mais elevados. Nesta situação, as Baixas apresentaram uma variação na produção oposta à das Altas. Em P1, os valores de proteína e de gordura no leite foram significativamente mais baixos que em P3, em ambos os grupos. A ureia no leite foi significativamente mais elevada nas Altas durante o P1, revelando potencial como biomarcador de stresse térmico. Do P1 para o P3 obser-vou-se uma redução gradual do hematócrito, da hemoglobina e da triiodotironina (T3). As Altas apresentaram uma maior redução de triiodotironina (T3) que as Baixas, como consequência de uma maior intensidade de aclimatação. Nos pelos não se registaram diferenças entre os períodos, o que contrasta com alguma bibliografia. Porém, a ausência da insolação direta poderá ter sido um fator determinante; ABSTRACT: The main objective of the present study was the characterization of anatomical and physiological variations that occur in the seasonal acclimatization process of dairy cows with high (Altas) and low (Baixas) milk yield potential. In this way it should be possible to do a selection of animals with good productive traits and also with adaptability indexes that allow a change in homeostasis to cope with the climatic conditions of the mediterranean climate. Meteorological, clinical, productive, physiological and anatomical data were collected. The study was conducted in an Alentejo's dairy farm, using 13 multiparous cows (6 with low milk yield and 7 with high milk yield), during three periods: P1 (animals acclimated to summer, in heat stress); P2 (animals acclimated to summer, thermoneutrality); P3 (animals acclimated to winter; thermoneutrality). In thermal stress (P1), the high milk yield group (Altas) shown greater thermolytic efforts and also higher heat storage. The milk yield in this group was also affected by heat stress, showing a decrease in production when the rectal temperature increased, with a delay of 24-42 hours. In this situation the Baixas group showed an opposite milk production variation. In P1, the protein and fat milk content was lower than in P3, in both groups. Milk urea levels were significantly higher during P1 in the Altas group, revealing potential as an heat stress biomarker. Hematocrit, hemoglobin and triiodothyronine (T3) values gradualy decreased from P1 to P3. T3 values were lower in Altas than in Baixas group, as a consequence of a more intense acclimatization. The hair analysis didn’t show the standard seasonal acclimatization process, indicating the absence of direct solar radiation as a determinant factor.

Thermoregulatory response in hair sheep and shorn wool sheep

This study’s main goal was to evaluate the thermoregulatory responses velocity through the variation of rectal temperature (RT), related to the thermolytic pathways, respiratory rate (RR) and sweating rate (SR) among different sheep breeds. Ninety female sheep, eighteen of each breed: Santa Ines and Morada Nova (Brazilian hair breeds), Texel, Suffolk and Ile de France (wool breeds) were challenged during three non-consecutive summer days (22◦42′S, 47◦18′W, and 570m of altitude, maximum air temperature of 33.5◦C, average relative humidity of 52±6.9%). The physiological variables were registered at 0800h (T1), 1300 h (T2: after 2 h of shade rest), 1400 h (T3) (after one hour of sun exposure) and in the shade at 1415 h (T4), 1430 h (T5), 1445 h (T6) and 1500 h (T7) and a thermotolerance index (TCI) was calculated as (10-(T7 to T4)-T1). The statistical analysis was performed by a mathematical model including the fixed effects of breeds and time frames, and the interaction between these effects, besides random effects such as animal and day. The Santa Ines breed presented the lowest RT after sun exposure (39.3 ± 0.12 ◦ C; P < 0.05) and it was the only one to recover morning RT 60 min after heat stress (38.7 and 38.9 for 1300 h and 1500 h; P > 0.05). Hair breeds presented RR lower (P < 0.05) than wool breeds. Although thick wool or hair thickness differs among and within hair and wool breeds (P < 0.05), SR did not differ among breeds and time (227.7 ± 16.44 g m−2 h−1 ; P > 0.05). The thermotolerance index did not differ among breeds, but it showed similar response (P > 0.05) 45 min or 1 h of shade after sun exposure. One week post shearing is not enough to wool breeds present to show thermotolerance similar to hair breeds.