A. Berman

Invited review: Are adaptations present to support dairy cattle productivity in warm climates?

Environmental heat stress, present during warm seasons and warm episodes, severely impairs dairy cattle performance, particularly in warmer climates. It is widely viewed that warm climate breeds (Zebu and Sanga cattle) are adapted to the climate in which they evolved. Such adaptations might be exploited for increasing cattle productivity in warm climates and decrease the effect of warm periods in cooler climates. The literature was reviewed for presence of such adaptations. Evidence is clear for resistance to ticks and tick-transmitted diseases in Zebu and Sanga breeds as well as for a possible development of resistance to ticks in additional breeds. Development of resistance to ticks demands time; hence, it needs to be balanced with potential use of insecticides or vaccination. The presumption of higher sweating rates in Zebu-derived breeds, based upon morphological differences in sweat glands between breeds, has not been substantiated. Relatively few studies have examined hair coat characteristics and their responses to seasonal heat, particularly in temperate climate breeds. Recently, a gene for slick hair coat has been observed that improved heat tolerance when introduced into temperate climate breeds. No solid evidence exists that hair coat in these lines is lighter than in well-fed warm climate-adapted Holsteins. Warm climate breeds and their F1 crosses share as dominant characteristics lower maintenance requirements and milk yields, and limited response to improved feeding and management. These characteristics are not adaptations to a feed-limited environment but are constitutive and useful in serving survival when feed is scarce and seasonal and high temperatures prevail. The negative relationship between milk yield and fertility present in temperate climates breeds also prevails in Zebu cattle. Fertility impairment by warm conditions might be counteracted in advanced farming systems by extra corporeal early embryo culture. In general, adaptations found in warm climate cattle breeds did not increase heat dissipation capacity, but rather diminished climate-induced strain by decreasing milk production. The negative relationship between reproductive efficiency and milk yield, although relatively low, also appears in Zebu cattle. This association, coupled with limited feed intake, acting over millennia, probably created the selection pressure for a low milk production in these breeds.

Mammary infection with Staphylococcus aureus in cows: progress from inoculation to chronic infection and its detection.

The progress of Staphylococcus aureus infection from inoculation to the early chronic stage was examined in 12 Israeli-Holstein cows (four primiparous and eight multiparous) for up to 48 d after inoculation. Before inoculation, the primiparous cows were free from any infection and the multiparous cows were infected by coagulase-negative staphylococci. Two quarters in each cow were inoculated intracisternally following milking with 2000 cfu of a local prevailing Staph. aureus strain, VL-8407. Infection was established in 21 out of 24 quarters. The control quarters remained free from infection during the study, with no significant change in function. No statistically significant differences were found between primiparous and multiparous cows in the responses examined. Somatic cell count (SCC) increased within 24 h of inoculation and remained high for the duration of the study. In the infected quarters mean ln (SCC) increased within 24 h from 9.9 +/- 0.5 before inoculation to 13.0 +/- 0.2 after inoculation; most of the cells were neutrophils. N-acetyl-beta-glucosaminidase activity, expressed as ln (nnmol/min per l), was increased from 0.9 +/- 0.6 to 2.4 +/- 0.2 by inoculation, and was highly correlated with SCC. The Staph. aureus count fluctuated with no particular relationship with SCC. The phagocytic activity of neutrophils was significantly lower in the inoculated than in the control quarters and this difference increased with time after inoculation. CD8+ T lymphocytes were the main subpopulation of lymphocytes found in inoculated quarters. After inoculation, maximum but not minimum electrical conductivity (EC) recorded during milking increased significantly. The rises in maximum EC varied significantly among cows. The rises in SCC were associated with a persistent increase in EC in only one of the eight cows examined. No clinical signs were observed, and milk yield and composition were not affected during the study period. The results suggest that some strains of Staph. aureus may induce a relatively mild response in mammary glands of cows in mid lactation, and that the concomitant development of such chronic Staph. aureus infections in two quarters may not be detected by changes in the EC of composite milk and in the yield of the cow.

Radiant heat loss, an unexploited path for heat stress reduction in shaded cattle

Reducing thermal radiation on shaded animals reduces heat stress independently of other means of stress relief. Radiant heat exchange was estimated as a function of climate, shade structure, and animal density. Body surface portion exposed to radiant sources in shaded environments was determined by geometrical relations to determine angles of view of radiation sources (roof underside, sky, sun-exposed ground, shaded ground) on the animals surface. The relative representation of environment radiation sources on the body surface was determined. Animal thermal radiation balance was derived from radiant heat gained from radiation sources (including surrounding animals) and that lost from the animal surface. The animal environment was assumed to have different shade dimensions and temperatures. These were summed to the radiant heat balance of the cow. The data formed served to estimate the effect of changes in intensity of radiation sources, roof and shaded surface dimensions, and animal density on radiant heat balance (Rbal) of cattle. Roof height effect was expressed by effect of roof temperature on Rbal. Roof underside temperature (35 to 75°C) effect on Rbal was reduced by roof height. If roof height were 4m, an increase in its underside temperature from 35 to 75°C would increase mean Rbal from -63 to -2 W·m⁻², whereas if roof height were 10 m, Rbal would only increase from -99 to -88 W·m⁻². A hot ground temperature increase from 35 to 65°C reduced mean Rbal heat loss from -45 to 3 W·m⁻². Increasing the surface of the shaded area had only a minor effect on Rbal and on the effect of hot ground on Rbal. Increasing shade roof height reduced the effect of roof temperature on Rbal to minor levels when height was > 8m. Increasing the roof height from 4 to 10 m decreased Rbal from -32 to -94 W·m⁻². Increasing indirect radiation from 100 to 500 W·m⁻² was associated with an increase in Rbal from -135 to +23 W·m⁻². Their combined effects were lower Rbal with increasing roof height and a reduction in rate of decrease with increasing level of indirect radiation. Roof height as an Rbal attenuator declined with increasing indirect radiation level. The latter factor might be reduced by lowering roof surface radiation absorption and through roof heat transfer, as well as by use of shade structure elements to reduce indirect radiation in the shaded area. Radiant heat from the cow body surface may be reduced by lower cow density. Radiant heat attenuation may thus further elevate animal productivity in warm climates, with no associated operation costs.

Subclinical mastitis assessed by deviations in milk yield and electrical resistance

Concurrent falls in milk production and electrical resistance of composite milk were examined in Israeli Holstein cows. The cows were milked three times a day by a system that recorded yield and the lowest electrical resistance in the composite milk from the four glands. The study included two groups: cows that experienced on day 0 a decline in resistance and milk production > or = 20% from the mean of the previous 9 d (62 cows, case group) and cows that experienced no such episodes over 9 d before and after a fixed day (118 cows, control group). Bacteriological status and somatic cell count (SCC) or California mastitis test scores were assessed on the fixed day in the control group, and on days 0, 1 and 2 in the case group. California mastitis test scores greater than 2 and SCC thresholds of 5 x 10(5) cells/ml were used to create two classes of leucocytosis. There was no statistically significant difference between the two groups in frequency distributions of pathogens and their types: in 30% of cows infection was not detected, 33% were infected by major pathogens (95% of which were Staphylococcus aureus), and 53.5% by minor pathogens (80% Micrococcus spp.). Cows in the case group had lower milk production during the 8 d following day 0. Mean electrical resistance was lower in infected cows and particularly in cows infected by Staph. aureus. High leucocytosis was associated with reduced electrical resistance in both groups, and was found in 93% of cows in the case group v. 25% in the control group. The results suggest that falls in electrical resistance of milk and in milk production were not linked to a specific pathogen, and were followed by 3-8 d of reduced milk production and electrical resistance. The study suggests that there are episodic aggravations in mammary health that do not evolve into clinical mastitis but may induce significant losses in milk yield and quality.

Forced heat loss from body surface reduces heat flow to body surface

Heat stress is commonly relieved by forced evaporation from body surfaces. The mode of heat stress relief by heat extraction from the periphery is not clear, although it reduces rectal temperature. Radiant surface temperature (Ts) of the right half of the body surface was examined by thermovision in 4 lactating Holstein cows (30 kg of milk/d) during 7 repeated cycles of forced evaporation created by 30s of wetting followed by 4.5 min of forced airflow. Wetting was performed by an array of sprinklers (0.76 m(3)/h), and forced airflow (>3m/s velocity) over the right side of the body surface was produced by fans mounted at a height of 3m above the ground. Sprinkling wetted the hind legs, rump, and chest, but not the lower abdomen side, front legs, or neck. The animals were maintained in shade at an air temperature of 28 degrees C and relative humidity of 47%. Coat thickness was 1 to 2mm, so Ts closely represented skin temperature. Mean Ts of 5 x 20cm areas on the upper and lower hind and front legs, rump, chest, abdomen side, and neck were obtained by converting to temperature their respective gray intensity in single frames obtained at 10-s intervals. Little change occurred in Ts during the first wetting (0.1+/-0.6 degrees C), but it decreased rapidly thereafter (1.6+/-0.6 degrees C in the fifth wetting). The Ts also decreased, to a smaller extent, in areas that remained dry (0.7+/-1.0 degrees C). In all body sites, a plateau in Ts was reached by 2 min after wetting. The difference between dry and wet areas in the first cooling cycle was approximately 1.2 degrees C. The Ts of different body areas decreased during consecutive cooling cycles and reached a plateau by 3 cooling cycles in dry sites (front leg, neck, abdomen side), by 5 cooling cycles in the hind leg, and 7 cooling cycles in the rump and chest. The reduction in mean Ts produced by 7 cycles was 4.0 to 6.0 degrees C in wetted areas and 1.6 to 3.7 degrees C in sites that were not wetted. Initial rectal temperature was 38.9+/-0.1 degrees C; it remained unchanged during first 5 cooling cycles, decreased by 0.1 degrees C after 7 cooling cycles, and decreased to 38.4+/-0.06 degrees C after 8 to 10 cooling cycles, with no additional subsequent decrease. The concomitant reduction in Ts in dry and wet areas suggests an immediate vasoconstrictor response associated with heat extraction and later development of a cooler body shell. The reduction in rectal temperature represents a response involving transfer of heat from the body core to the body shell. This response mode requires consideration in settings of heat stress relief.