Omega ratio in ewes affects sex ratio in their lambs

The diet a producer feeds their ewes at joining can affect the proportion of male to female lambs, says NSW Department of Primary Industries' Dr Ed Clayton.

Dr Clayton, a livestock research officer for ruminant nutrition at Wagga Wagga and guest speaker with the Grains Research and Development Corporation at the Henty Machinery Field Days in September, said a ewe's diet could influence not just the sex of her offspring, but also go on to influence the reproductive performance of those lambs later in their life, as well as the ratio of fatty acids, omega 6 to 3, in the lamb's meat.

Feeds such as pasture, especially green pasture, and silage, where typically higher in omega 3 fatty acids, while grains tended to contain proportionately higher levels of omega 6.

The ratio of these fatty acids in a ewe's feed in the lead-up to and during joining influenced the ratio of these fatty acids in the ewe's blood.

This in turn appeared to correlate with the sex ratio of lambs produced, as well as with the reproductive performance of those lambs in later life, Dr Clayton said.

"In the trial, we looked at omega 3 and omega 6 in sheep and at their effect on prostaglandin, as this is involved in a lot of processes to do with reproduction, onset of oestrus and ovulation, as well as parturition," Dr Clayton said.

"We looked at the sex ratio of lambs and also what happens to the progeny of those ewes.

"If you can get more omega 3 in the blood, you get more males; less omega 3 in the blood, we get more females."

In the Wagga trials, the omega 3 diet was based on silage, and the omega 6 diet on oats and cotton seed meal.

"The ratio of the two fatty acids (omega 6:omega 3) is what we are really interested in, so in the silage diet it was about one-to-one, and in the grain based diet it was about 13:1," he said.

In the lambs, this correlated to a 50:50 sex ratio among the silage ration group.

With the grain diet, the proportion of females increased by 14-15pc.

The trial rations were fed for six weeks leading up to, and three weeks after joining.

"We looked at first-cross ewes and Merino ewes and we got some really consistent differences in the proportion of females, up to 30pc more females (in some cases) with our oats and cottonseed meal diet," Dr Clayton said.

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However, he said it was difficult to trust reproduction research "because it changes all the time... so we conducted a large number of controlled studies over three or four years", he said.

In addition, 11 on farm studies were conducted in conjunction with Australian Wool Innovation and Meat and Livestock Australia.

In these trials, ewes grazed pasture (omega 3 diet), or pasture supplemented with 600 grams a head of oats a day (omega 6).

"We often saw a flushing effect when we fed grain, which was expected. We didn't see as consistent a difference in sex ratios as with our control trials, but what we did see was a similar relationship with overall diet," Mr Clayton said.

"So if our overall diet was higher in pasture, we got less females and if the overall diet is higher in grain, we got more females."

In other trials, such as those at Wagga, the diet effect was observed to continue into the second generation.

"We had some really interesting effects on the second generation. We fed these ewes for six weeks leading up to joining and then all our ewes were treated the same during pregnancy and lactation (i.e. they all went back on to the control ration three weeks after joining)," he said.

"We looked at what happens with the fatty acid of the lambs at birth and then the metabolism and reproduction rate during their life."

He said the ewes' diet leading up to and during joining influenced the omega ratios in the lambs, with those from the grain-fed ewes accumulating less omega 3 through their lifetime.

"Even when all the ewes were fed the same diet throughout the rest of pregnancy and lactation, the lambs from the grain-fed dams actually accumulated significantly less omega 3. It changed their metabolism," he said.

"We also have a study from Cowra showing that we've changed the expression of genes associated with the metabolism of those fatty acids.

"So what we fed the mums has flowed through to what happens with the lambs."

Dr Clayton said they also looked at the levels of omega 3 in the meat, because of associated health status claims.

"When we took wether lambs and fed them algae to try and bump up that omega 3 again, we got a significant difference in the accumulation of that omega 3 depending on what their mums were fed (i.e silage or grain).

The lambs from the grain-fed ewes accumulated less omega 3 in their meat compared to the lambs from ewes fed silage.

The ewe lambs from the grain-fed ewes, however, demonstrated a higher reproductive potential, with 25pc more fetuses at scanning.

"And that flowed through for four joinings for this cohort of lambs," he said.

"It wasn't a difference in the proportion of ewes pregnant, just the number of fetuses."

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