The negative effect of heat stress on dairy cows, with a temperature humidity index (THI) over 72, has been extensively studied. However, there are few studies on THI values under 72 that compare the effect of heat stress in different lactation stages. The objective of this study was to determine the heat stress effect on two lactation stages with a THI below the threshold 72. Thirty-nine multiparous, non-pregnant Holstein cows with more than 30 kg/cow.day of solid-corrected milk were used in a randomised complete block design to evaluate six treatments. The experimental design had a 3 by 2 factorial arrangement with three heat stress mitigation strategies: No shade (CON), access to shade only, and access to shade combined with sprinkling and ventilation, and two stages of lactation: early (S1) and late (S2), 12 ± 10.3 and 201 ± 45.8 days in milk respectively, for a duration of 81 consecutive days. All treatments, except CON had access to artificial shade from 0900 hours to 0500 hours next day. From 0500 hours to 0900 hours all cows were managed together in a grazing session. Additionally, sprinkling and ventilation cows had two 30-min sessions of ventilation and spray (0900 hours, 1530 hours). The average THI was 70.1 ± 4.46 (minimum THI: 60.4; maximum THI: 81.7) and the average hours above 72 were 7.8 ± 5.98. Animals in S1 presented higher solids-corrected milk reduction (P < 0.0001; 5.4 and 1.9 kg/cow.day), and protein yield (<0.0001; 0.13 and 0.54 kg/cow.day) than animals in S2 when shade was not allowed (CON). It was concluded that under these conditions, animal productivity is more negatively affected in early lactation animals. The use of shade with or without spray and ventilation mitigates heat stress effects on both stages of lactation.
El objetivo de este trabajo fue caracterizar la ocurrencia de condicionantes meteorológicas (CM) estivales de diferente severidad en Salto (Uruguay) y su impacto sobre las respuestas fisiológicas y productivas de vacas lecheras. Se caracterizaron las CM según el índice de temperatura y humedad (ITH) promedio, horas al día con ITH ≥72 y las temperaturas mínima y máxima diaria. En los animales, se registró la temperatura rectal (TR) y la frecuencia respiratoria (FR) a las 6:00 y 15:00 horas, también el tiempo dedicado al pastoreo (TP) durante el día. Se identificó una ola de calor severa (OCS), tres olas de calor leve (OCL) y días de no ola de calor (NOC). La FR y la TR de las vacas variaron en función de CM (P<0,0001). Las FR y TR(6:00 y 15:00) fueron mayores en OCS con respecto a OCL y a NOC (P><0,0001). El TP fue menor durante OCS con respecto a OCL y NOC (P><0,0001). La producción de leche, grasa y proteína fueron menores en OCS (P><0,0001) con respecto a OCL y NOC. El recuento de células somáticas fue mayor en OCS que en OCL, pero no que en NOC (P=0,0618). Durante OCS las vacas no lograron mantener la normotermia y tuvieron menor TP, reduciendo la productividad. En OCL las vacas recuperaron la normotermia durante la noche, no reduciendo la productividad en comparación a los días NOC.> <0,0001). Las FR y TR(6:00 y 15:00) fueron mayores en OCS con respecto a OCL y a NOC (P <0,0001). El TP fue menor durante OCS con respecto a OCL y NOC (P <0,0001). La producción de leche, grasa y proteína fueron menores en OCS (P <0,0001) con respecto a OCL y NOC. El recuento de células somáticas fue mayor en OCS que en OCL, pero no que en NOC (P=0,0618). Durante OCS las vacas no lograron mantener la normotermia y tuvieron menor TP, reduciendo la productividad. En OCL las vacas recuperaron la normotermia durante la noche, no reduciendo la productividad en comparación a los días NOC.
Adaptative traits (rectal temperature-RT, respiratory rate-RR) and grazing behavior (Grazing, Ruminating and Rest time, and Sun or Shade time) of Bonsmara-Hereford crossbred-BH, n = 15, and purebred Hereford-HH, n = 18, yearling heifers, in a grazing system of Uruguay. Environment characterization was made using THI adjusted by radiation and wind speed (no, mild, and severe heat weaves), and the comprehensive climatic index-CCI (no, middle, moderate and severe stress). Adaptative traits were measured twice a day, weekly, in two consecutive days in summer and winter at 08h00 and 16h00. Grazing behavior was observed from 07h00 to 21h00 each 600 s twice in summer. The records were analyzed using a mixed model. Significant effect of genotype was observed in mild heat waves for RR and RT in the afternoon (BH lower than HH). For CCI in a.m., the RR was lower than HH in BH in severe stress, while in p.m., the RR was lower in all of CCI levels. RT in p.m. in moderate and severe was lower in BH than in HH. In winter, no differences were found. In grazing behavior, HH rests longer than BH doe; also, HH spends more time in the shade (34 %) than BH does (22 %). BH genotype showed better thermoregulation and grazing behavior at higher temperatures compared to HH.
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