22 August 1997

Embryo mortality linked

to higher protein intake

Manage fertility successfully and productivity will improve. Our special focus examines how to improve breeding success with autumn calving suckers, heifers, and when AIing pigs. But first Emma Penny finds out how diets high in protein could be increasing embryo mortality and infertility

THE sudden introduction of rations high in rumen degradable nitrogen may adversely affect embryo survival and development.

Thats the conclusion drawn from latest research by SAC animal and feed technology scientists John Robinson and Tom McEvoy, Craibstone, Aberdeen.

The trials looked at the effect of dietary excesses of urea on viability and metabolism of sheep embryos before implantation. However, both stress that the research could equally be applied to cattle, as Dr Robinson explains.

"Conception rates in cattle tend to dip after turnout. It may be that lush spring grass, which has had plenty of N applied, is high in rumen degradable N.

"Rumen degradable N is broken down to ammonia by rumen bacteria. Ammonia is then usually converted in the liver to less harmful urea. It may, however, be the case that there is a lag time between the increase in ammonia and the production of enough liver enzymes to detoxify it."

That lag time – which exceeded the two week trial period – means that not all the ammonia is removed from the animals system. This causes blood ammonia levels to rise, as Dr McEvoy explains.

"Ammonia has a vast capacity to change pH levels, and it is known to be toxic to cells. It seems to change the metabolism of embryos, with few surviving."

According to Dr McEvoy, the change in embryo metabolism – up-regulation – will kill many of the embryos, but those which survive are likely to give lethargic offspring with heavier birthweights.

In the first trial at Aberdeen, the effect of increased dietary urea on survival and metabolism of embryos from Border Leicester x Scottish Blackface ewes was examined. Those on the high urea diet received 24% crude protein rations – an extra 9% protein in the diet – for 12 weeks.

Ewes were placed on the diet, sponged and inseminated, and embryos collected from some ewes five days after insemination and others after 11 days. One embryo was returned to each ewe, while the others were grown in vitro.

None of the in vitro embryos from the high crude protein diet were viable. But one of the high urea embryos returned to the ewe produced a lamb. It was 2.5kg heavier than those on the control ration, and 1.8kg heavier than lambs from embryos on the low urea diet.

Uniform environment

In the second trial, control and high urea diets were fed to ewes which were sponged, inseminated and flushed. The embryos were put into a uniform control environment to see whether the metabolism of the high urea embryos would re-adjust.

Despite being put in the control environment, embryos from ewes on the high urea ration had a metabolism up to 2.8 times greater than embryos from ewes on the control diet, says Dr McEvoy.

"This shows that once embryos have been set on an up-regulated course they continue on it. The embryo either dies or survives to produce an over-size offspring. Ammonia can stimulate growth in small amounts, but in excess it appears to cause embryo burnout."

Results of both trials show that producers must think far more carefully about management around the time of conception, warns Dr Robinson.

"When turnout onto grass which has received a lot of fertiliser and bulling coincide there may be fertility difficulties. This might be particularly so where high genetic merit cows are involved and there is little slack in the system.

"In Ireland, for example, a lot of effort has gone into achieving early turnout; cows are pushed onto grass high in degradable N, which may well affect embryo survival.

"We dont know at this stage quite what the critical level of rumen degradable protein is, or how long the lag period is before the liver produces enough enzyme to convert the excess ammonia."

However, Dr Robinson suggests that fishmeal may play a role in providing key amino acids which help detoxify ammonia, in addition to supplying fatty acids which aid embryo survival.

However, he adds that more work must be done. "We need further investigation to help understand fertility better."n

Spring grass which has received a lot of N may affect fertility, warn John Robinson and Tom McEvoy.

HIGHNTRIAL

&#8226 Lower embryo survival.

&#8226 Increased embryo metabolism.

&#8226 Produce oversize offspring.