Because there is a  Negative correlation between mlk yoeld and fat percentage

Generally tropical dairy breeds are fed with between 55 and 85% forage and 15 to 45% of concentrate. Such rations, high fiber, promote high acetate/butyrate ruminal fermentation patterns, and thus precursors to produce as much milk fat

but we get low fat when fed temperate cows on the same ration...

I think it is influenced by genetic and metabolic pathways,,,,,, but how?

Of the temperate dairy cattle breeds Jersey and Guernsey cows produce milk with the highest fat content (4.5 – 5.0 %) compared with breed such as Holstein.  Therefore, I also believe this character is related to genetics.

Hi Eltahir, You have all the answer in fernando reply. Genetical difference is evident  if you speak about any local breed and especially the fact that promoting milk production in dairy cattle select lactose (and caseins in less extend) production that regulate milk volume thus decrease milk fat percentage and global milk richness. Even between the holstein 1970 and the holstein 2015 you can see the milk richness reduction due to milk volume increase (called inappropriately "dilution" effect). You have also a possible effect of diet and of temperature and water evaporation/intake.

Hi Eltahir, If it is native races, the difference may be genetic (allele B kappa casein). We have observed in a local breed, a greater frequency of this allele, compared to foreign breeds.

dear Nyala, make no mistake about it, the bigest influencing factor is simply level of milk production, not genetics. Put another way, the relationship between yield and contents will simply override any genetic control- on this score. It is that basic. I dont know what breed you are working with, however in west africa this fact has been well researched with  the White Fulani cattle, which also named BUNAJI. You may look up the literature. 

Hi Everybody.  You can´t  talk about performance differences only from the environmental side, not speaking from genetics side. Genetic of tropical livestock is very different to temperate livestock and for that occur the differences that  Belkys V. says

Juan Carlos, you are right genetic is basic to almost everything. It is a fact that even among temperate breeds there is genetic difference in milk fat output. However the basic issue why tropical breeds have high fat content relative to protein is just the low milk yield. When you control for genetics, nutrition, parity etc, milk yield and fat content are strongly anticorrelated all over the world. If you do the calculations by simply basing fat level on milk output, you will be able to prove this to be true. An almost identical story exists in poultry. With high egg yield in number you are likely to obtain lower egg weight, and vice versa. that is just the biology of production. Therefore to have an objective basis for comparisons, we do the maths called Egg Mass- to account for both number and weight.  

You might argue why not increase the milk yield of tropical dairy breeds, by working on the nutrition etc,? The genetics already placed on limit on how far they can go! But this in itself may not be a disadvantage, as there could be an evolutionary basis/need for that- we dont know. But it is important we dont always take low production as a disadvantage. We may still need those genes sometimes, someday. This is what some guys have been working on apparently, for donkey years. 

I agree with Gustavo. Tropical cattle (meaning cattle in tropical countries) are most of the time fed with forages that are high in fibre. Such rations promote the production of acetate which is essentially a precursor for production of fat in milk. Genetics too may be at least partly responsible.  

Genetic definitely mark directs the flow of nutrients in the animal in different ways. But it is quite true that tropical breeds adapted to high temperatures and high in fiber and low nonstructural carbohydrate fodder, with fermentation patterns and volumes have  higher digestive tract (rumen volume), allowing in association with microorganisms fermentation patterns acetic/ butyric, precursors produce more fat instead of lactose, therefore generally produce more fat in milk. This could be explained with genetically more of enzymes and / or promoting hormones synthesis of fat compared to temperate races.

Genetics and forrage quality explain this big difference... 

this is a really nice question. prepares this is the genetics of animals that vary with the change in climate. i dont think metabolic pathways will varry a lot. it is mostly some kind of adoptive changes that is reflected in change in the milk yield. for an instance the DM intake will reduce with rise in temperature of tropical climate so there is a drop in milk it ... and may cause a hike in fat %.

along with this to validate the role of genetics it is better to check both the animals in the same conditions. again long term trial needed to see the adoptive changes among the animals. but both the animals reared in the same climatic conditions for a long duration may solve some of the puzzles related to this topic.

Generally tropical dairy breeds are fed with forages and it's known that the fiber promotes the acetate/butyrate ruminal fermentation and those are directly related with the fat in milk. The other possibility is It can also have some relations with the breeds and the production of milk, which can have a "concentration yield" of fat. It's like: tropical dairy breed produces the same content of fat than temperate breed but, because the yield of milk production is lower in tropical breed, the % looks higher.

The main reason is that dairy cattle which  live in tropic produce less milk than counterparts that live in moderate or cold climate

Our Tropical Dairy Cattle "Carora". This tropical race is capable of producing more than 35 lt / day, 290 days of lactation, dry environment where temperatures exceed 40 ° C, consuming as part of its fibrous ration of medium to poor quality forages.

https://twitter.com/BiominbloqCA/status/662772750960971776

https://twitter.com/BiominbloqCA/status/662777236412592128

Dear Gustavo

Very interesting information.

Are these figure you gave means of the breed or individual records?

Genetics (breed effect), nutrition, and climate are responsible. The tropical dairy breeds consume more of fibrous feeds which result in lower propionate but higher acetate/butyrate production. Both acetate and butyrate are lipogenic and thus favour production of more milk fat at the  expense of milk yield, which is inversely related to milk fat. Similarly, high temperature in the tropics limits feed intake which affects milk yield of even high producing temperate breeds in the tropics. As milk yield drops, butterfat increases. Even when the feeding systems are the same for the same temperate dairy breeds in the temperate and in the tropics, environmental effect, chiefly temperature, will still impact milk fat due to reduction in milk yield. Thus nutrition and the environment appear to be the major factors as long as the same breed of dairy cattle is used in both regions, and temperature becomes more critical when the same feeding system/strategy is applied in both regions to the same dairy breed.