Milk production of friesian cows subjected to high daytime temperatures when allowed food eitherad lib or at night-time only

Abstract

Four lactating Friesian cows housed in climate chambers were exposed to the following sequential treatments: I, a four-week period when they were allowed constant access to an ad lib supply of a complete diet and water in a thermoneutral environment (ambient temperature (Ta) 14 to 20 degrees C and relative humidity (r.h.) 50 to 75%); II, a five-week period during which they were exposed to Ta 38.5 degrees C/r.h. 85% for 61/2 h during daytime and Ta 14 to 20 degrees C/r.h. 50 to 75% for the remainder of each day with constant access to ad lib complete diet and water; III, a four-week period under the same environment as treatment II but with access to food restricted to between 19.00 and 07.00; IV; a four-week period under the environment and feeding regime described for treatment II. The voluntary food intake (VFI) of cows subjected to II was not depressed as a result of high daytime temperature despite the fact that they exhibited daytime clinical features indicative of heat stress; this was because of a 27.8% compensatory increase in night-time feeding. Nevertheless there was an accompanying significant increase in the rate of lactation decline of 8.7% per month greater than that normally expected under the thermoneutral conditions (I). Subsequent restriction of food access to night-time (III) resulted in a significant reduction in mean VFI of 7.5%; however, the rate of lactation decline was drastically reduced to a value not dissimilar to the natural decline of 3.97% per month observed under thermoneutral conditions (I).

Effect of heat stress on cows's hormonal status in the lactation period

“…Visokomlečne krave nisu aklimatizovane na naglo povećanje spoljašnje temperature (akutni toplotni stres) i ubrzan rad srca je neposredan odgovor organizma na stanje stresa (Richards 1985).…”

“…Do povećanja telesne temperature može doći i pri optimalnim uslovima spoljašnje temperature pri čemu dolazi do prolaznog ili trajnog blokiranja aktivnosti homeostatskih mehanizama za regulisanje telesne temperature, što je čest slučaj kod bakterijskih i virusnih infekcija. Sa porastom ambijentalne temperature odavanje toplote kondukcijom, konvekcijom i radijacijom se smanjuje kod visokomlečnih krava (Berman i sar., 1985;Maia i sar., 2005) a oslobađanje viška toplotne energije organizma putem znojenja i isparavanja vode sa sluzokože organa za disanje sve više dobija na značaju (Richards, 1985). Berman i sar.…”

“…U uslovima toplotnog stresa kod visokomlečnih krava dolazi do prolaznog povećanja telesne mase, koji je rezultat prevelike količine konzumirane vode (Richards, 1985). Visoka specifična toplota vode omogućava da deo toplotne energije preuzme uneta voda u organizam, koju su životinje popile u toku dana, a da tu energiju odaju u toku noći kada je značajno niža spoljašnja temperatura (Schmidt-Neilsen, 1964).…”

“…Bianca (1965) je prva zabeležila pad mlečnosti od 33% pri izlaganju krava uticaju hroničnog toplotnog stresa (35°C), dok Richards (1985) obaveštava da krave držane danju pod uticajem toplotnog stresa, ali pri nižim noćnim temperaturama (25°C), nisu smanjile proizvodnju mleka. Pad proizvodnje mleka je najverovatnije posledica negativnog delovanja toplotnog stresa na mehanizme koje kontrolišu sekretornu sposobnost mlečne žlezde (Silanikove 1992).Visokomlečne krave su najosetljivije na delovanje toplotnog stresa zbog čega i klimatski činioci imaju najveći uticaj na proizvodnju mleka upravo u periodu tokom prvih 60 dana laktacije (Sharma i sar., 1983).…”

“…Mnogi autori obaveštavaju o smanjenju proizvodnje mleka u uslovima povišene spoljne temperature (Thatcher 1974;Johnson, 1976;McDowell i sar., 1976;Richards 1985;Silanicov, 1992;Kadzere i sar., 2002;West, 2003). U našem radu najviša dnevna mlečnost bila je u periodu optimalnih ambijentalnih uslova (period C) i iznosila je 43,08 L po kravi.…”