Behavior of lactating dairy cows under mild and severe heat stress with free access or not to shadow

Abstract Text:  In the tropics, during several months of the year, air temperatures are well above the upper critical temperature for lactating dairy cattle. Despite this fact, many dairy farmers still do not provide shade. The objective of this trial was to study the behavior of dairy cows with or without access to shadow under mild (afternoon THI =74.7) and severe heat stress (afternoon THI=81.2). Twenty-eight Holstein cows in mid-lactation producing 21±3 L per day were used. Cows were split into four groups according to heat stress intensity (mild and severe) and access to shadow (with and without). Behavior of all cows was visually observed from 6 am to 9 pm for 4 days and time spent grazing, ruminating, idling, walking, lying and number of water ingestion bouts were recorded. Body temperature and respiratory rate were recorded at 6 pm. Data was submitted to analysis of variance according to a completely randomized design considering main effects of heat stress severity (n=2), access to shade (n=2) and their interaction.  Respiratory rate was greater under severe compared with mild heat stress (100.8 x 72.8, P<0.001) and when no shadow was provided compared with free access to shadow (98.9 x 74.7, P<0.001). Significant interactions were detected for time spent ruminating, grazing, walking, idling, lying and for body temperature. Shadow was beneficial even under mild heat stress, as cows spent less time walking and spent more time ruminating, but provision of shadow did not improve time spent grazing, idling or lying. Under severe heat stress, provision of shadow helped cows in reducing walking time and allow acceptable values for time spent grazing, idling and lying besides body temperature, but it was unable to keep ruminating time. Dairy cattle should have free access to shadow independently of severity of heat stress.

 

				Heat stress
Attribute	P>F	P>F	P>F (interaction)	Mild		P>F	Severe		P>F
	Heat stress severity	Access to shadow	Heat stress x access to shadow	with shadow	without shadow		with shadow	without shadow
Walking (min)	0.0140	0.0001	0.0001	19.6	31.1	0.0323	12.1	52.0	0.0001
Ruminating (min)	0.0001	0.3382	0.0001	175.4	142.3	0.0021	13.1	61.0	0.0001
Grazing (min)	0.3308	0.0001	0.0001	219.0	232.0	ns	257.9	169.2	0.0001
Idling (min)	0.0001	0.1926	0.0436	192.3	201.8	ns	271.0	230.1	0.0243
Lying (min)	0.0054	0.0001	0.0001	158.6	151.8	ns	186.7	50.0	0.0001
Body temperature (°C)	0.0072	0.0001	0.0112	39.6	40.0	0.0298	39.64	40.68	0.0001

Keywords:  behavior, heat stress, shadow