Association of Phenotypic Markers of Heat Tolerance with Australian Genomic Estimated Breeding Values and Dairy Cattle Selection Indices.

Dairy cattle predicted by genomic breeding values to be heat tolerant are known to have less milk production decline and lower core body temperature increases in response to elevated temperatures. In a study conducted at the University of Melbourne's Dookie Robotic Dairy Farm during summer, we identified the 20 most heat-susceptible and heat-tolerant cows in a herd of 150 Holstein Friesian lactating cows based on their phenotypic responses (changes in respiration rate, surface body temperature, panting score, and milk production). Hair samples were collected from the tip of the cows' tails following standard genotyping protocols. The results indicated variation in feed saved and HT genomic estimated breeding values (GEBVs) (p ≤ 0.05) across age, indicating a potential for their selection. As expected, the thermotolerant group had higher GEBVs for HT and feed saved but lower values for milk production. In general, younger cows had superior GEBVs for the Balanced Performance Index (BPI) and Australian Selection Index (ASI), whilst older cows were superior in fertility, feed saved (FS), and HT. This study demonstrated highly significant (p ≤ 0.001) negative correlations (-0.28 to -0.74) between HT and GEBVs for current Australian dairy cattle selection indices (BPI, ASI, HWI) and significant (p ≤ 0.05) positive correlations between HT and GEBVs for traits like FS (0.45) and fertility (0.25). Genomic selection for HT will help improve cow efficiency and sustainability of dairy production under hot summer conditions. However, a more extensive study involving more lactating cows across multiple farms is recommended to confirm the associations between the phenotypic predictors of HT and GEBVs.

Impact of Heatwaves on the Physiology and Retail Meat Quality of Lambs.

The experiment investigated the impact of heatwaves (HWs) on the physiology, postmortem muscle metabolism and meat quality of lambs. Seventy-two second-cross lambs (Poll Dorset × (Border Leicester × Merino)) were selected and exposed to either 1, 3 or 5 days of HWs or thermoneutal (TN) (28-38 °C and 40-60% relative humidity, RH; 18-21 °C, 40-55% RH) conditions in climate-controlled chambers. Lambs exposed to 1-5 days of a HW exhibited higher respiration rates (RRs), rectal temperatures (RTs), skin temperatures (STs) and heart rates (HRs) compared to lambs exposed to an equal duration of TN conditions. However, HWs had no significant effects on muscle metabolism (rate and extent of pH decline, muscle glycogen and lactate content) and meat quality (cooking loss and shear force). Similarly, there were limited impacts of 1-5 days of HW on the colour (L*, a*, b* and R630/580) and drip loss of the longissimus thoracis et lumborum (LTL) and semimembranosus (SM) muscles for 4 days' overwrap retail display. Results suggest that short-duration HWs (1-5 days) had significant negative effects on animal physiology but had no effect on the muscle metabolism and meat quality.

Impact of heat stress on the growth performance and retail meat quality of 2nd cross (Poll Dorset × (Border Leicester × Merino)) and Dorper lambs.

The present study investigated the impact of heat stress and genetics on lamb growth performance and meat quality. Forty-eight Dorper and 2nd cross [Poll Dorset × (Border Leicester × Merino)] lambs (38--42 kg; 4-5 months old) were allocated to either thermoneutral [TN; 18-21 °C, 45-55% relative humidity (RH)], or heat stress (HS; 28 °C-38 °C, 40-60% RH) conditions in a 2 × 2 factorial design for 2 weeks. Compared with 2nd cross, Dorper lambs had a lower respiration rate (RR) and rectal temperature (RT), and exhibited less decline in body weight under HS. 2nd cross lambs showed a higher body weight gain than Dorpers under TN conditions. HS increased a* and chroma of the Longissimus thoracis et lumborum (LTL) from 2nd cross lambs over 10 days of display, but had no impact on Dorper LTL. In conclusion, Dorpers showed higher heat tolerance compared with 2nd cross lambs during the 2 weeks HS.

Effect of slaughter age and post-mortem days on meat quality of longissimus and semimembranosus muscles of Boer goats.

This study investigated the effects of age of animal and days post-mortem (PM) on meat quality of Boer goats. Twenty-four (24) wether Boer goats of two age groups (2YO group: 2 years old and 9MO: 6-9 months, with 12 animals/group) were slaughtered in a commercial processing plant. The pH@Temp18 was estimated to be above 6 in both age groups with higher (P < 0.01) values in 2YO goats. The PM storage for 14 days reduced the shear force in both age groups (P < 0.01). 2YO goat muscles (longissimus and semimembranosus) exhibited higher (P < 0.01) Thiobarbituric acid reactive substance values (TBARS), indicating increased lipid oxidation. Glycogen (P < 0.01) and lactate content (20 min post-slaughter) in longissimus of 9MO were lower compared to 2YO, and total muscle glycogen concentration was lower (P < 0.01) in both age groups below the threshold levels. Hence, as hypothesized, age and days PM proved to play crucial roles on Boer meat quality.

A Meta-Analysis of the Effectiveness of High, Medium, and Low Voltage Electrical Stimulation on the Meat Quality of Small Ruminants.

The current study is a meta-analysis of the effects of electrical stimulation (ES, n = 28 papers) with different voltages combined with different ageing periods (0-3, 4-7, and 8-14 days) on the meat quality of small ruminants. A comprehensive search for published studies on meat quality of small ruminants investigating the application of low, medium, and high voltage electrical stimulation, was performed using Google Scholar, ScienceDirect, PubMed, and Scopus databases. Forest plots, funnel plots, and other statistical tools and tests were used in the study to analyze the results. Electrical stimulation significantly reduced ultimate pH (p < 0.001), Warner-Bratzler shear force (WBSF) (p < 0.001), cooking loss (p < 0.05), and purge loss (p < 0.001). In addition, sarcomere length (p < 0.01), myofibrillar-fragmentation index (MFI) (p < 0.001), and color (L*, a*, b*) (p < 0.001) showed higher values in meat subjected to ES as compared with the control group. In conclusion, the meta-analysis revealed statistical proof of beneficial effects of ES on meat quality of small ruminants in terms of ultimate pH, tenderness, enhanced proteolysis, and higher colorimetric values.

Heat Stress Impacts on Lactating Cows Grazing Australian Summer Pastures on an Automatic Robotic Dairy.

The objective of this study was to measure the impacts of summer heat events on physiological parameters (body temperature, respiratory rate and panting scores), grazing behaviour and production parameters of lactating Holstein Friesian cows managed on an Automated Robotic Dairy during Australian summer. The severity of heat stress was measured using Temperature-Humidity Index (THI) and impacts of different THIs-low (≤72), moderate (73-82) and high (≥83)-on physiological responses and production performance were measured. There was a highly significant (p ≤ 0.01) effect of THI on respiratory rate (66.7, 84.7 and 109.1/min), panting scores (1.4, 1.9 and 2.3) and average body temperature of cows (38.4, 39.4 and 41.5 °C), which increased as THI increased from low to moderate to high over the summer. Average milk production parameters were also significantly (p ≤ 0.01) affected by THI, such that daily milk production dropped by 14% from low to high THI, milk temperature and fat% increased by 3%, whilst protein% increased by 2%. The lactation stage of cow had no significant effect on physiological parameters but affected (p ≤ 0.05) average daily milk yield and milk solids. Highly significant (p ≤ 0.01) positive correlations were obtained between THI and milk temperature, fat% and protein% whilst the reverse was observed between THI and milk yield, feed intake and rumination time. Under moderate and high THI, most cows sought shade, spent more time around watering points and showed signs of distress (excessive salivation and open mouth panting). In view of the expected future increase in the frequency and severity of heat events, additional strategies including selection and breeding for thermotolerance and dietary interventions to improve resilience of cows need to be pursued.

Comparative Assessment of Thermotolerance in Dorper and Second-Cross (Poll Dorset/Merino × Border Leicester) Lambs.

The objective of this study was to compare the thermotolerance of second-cross (SC; Poll Dorset × Merino × Border Leicester) and Dorper lambs. Dorper and SC lambs (4-5 months of age) were subjected to cyclic heat stress (HS) (28-40 °C). The temperature was increased to 38-40 °C between 800 and 1700 h daily and maintained at 28 °C for the remainder of the day (30-60% relative humidity (RH)) in climatic chambers for 2 weeks (n = 12/group), with controls maintained in a thermoneutral (TN) (18-21 °C, 40-50% RH) environment (n = 12/group). Basal respiration rate (RR), rectal temperature (RT) and skin temperature (ST) were higher (p < 0.01) in SC lambs than in Dorpers. HS increased RR, RT and ST (p < 0.01) in both genotypes, but the levels reached during HS were lower (p < 0.01) in Dorpers. HS increased (p < 0.01) water intake to a greater extent in SC lambs, while feed intake was reduced (p < 0.05) by HS in SC lambs but not in Dorpers. HS increased (p < 0.01) blood urea nitrogen and creatinine in SC lambs only. Plasma non-esterified fatty acid concentrations were reduced (p < 0.05) by HS in SC lambs but increased (p < 0.05) in Dorpers. There was no effect of HS on pO2, cHCO3- and cSO2, but higher (p < 0.01) blood pH and lower (p < 0.01) pCO2 were recorded under HS in both genotypes. Blood electrolytes and base excess were reduced (p < 0.01) under HS, while a genotype difference (p < 0.05) was only observed in blood K+ and hemoglobin concentrations. Basal plasma prolactin concentrations were lower (p < 0.01) in Dorpers but were elevated at a similar level during HS (p < 0.01) in both genotypes. Dorper lambs are more resilient to HS than SC lambs. Future research should focus on confirming whether the better heat tolerance of Dorpers is translated to better returns in terms of growth performance and carcass traits over the summer months.