Beef improvement programs have traditionally focused on genetic selection. Recent advances in genomics has accelerated selection by allowing us to identify key differences in the DNA of an individual animal’s DNA sequence. Wow! — being able to “look” right into an animal’s DNA. You’d think that there can’t be much left to discover in the field of genetics. But as we understand more of the science, additional complicating factors are coming to light.
The standard view of how an animal’s performance is determined takes into account two main factors: its genetic makeup and the environment it is in, including nutrition, housing, climate, disease etc.
The expression of the animal’s genes, interacting with its environment results in its performance for each trait. For beef cattle production, the traits we are interested in include birth weight, growth rate, feed efficiency, carcass attributes and fertility, among others. But we are finding out that the expression of those genes is highly variable.
One of the foundation principles of modern genetics is that an animal’s genes set its ultimate potential to perform in any given environment. For example, a feedlot steer on a given plane of nutrition can perform up to a maximum level set by its genetic makeup. If its environment during the feeding period is optimum (including health status, adequate bedding and space, freedom from stress etc.) then the steer’s performance will only be limited by its genetic potential. And since genetic makeup is determined at conception, its future potential is set “in stone,” and would only be affected by the environment at the time the trait was expressed. Or is it?
Recent research shows that the future performance of an animal can be affected by the maternal environment of the fetus during its early development. And this may occur even though traits expressed by the newborn animal, such as birth weight, are unaffected! This concept is called fetal programming, and has important implications for beef production systems.
For example, the weight and body condition of pregnant cows, which affects fetal nutrition, is dramatically impacted by the plane of nutrition. This plane can vary from year to year on the same farm, depending on the growing and harvesting conditions for the forages which make up the majority of most beef cow diets. As well, the seasonality of the beef cow production cycle sees major diet changes, as many cows transition from grazing to stored feeds during early gestation.
The need for additional dietary feed energy as temperatures decline from fall into winter further complicates things. The end result is that pregnant cows often undergo significant changes in body weight and fat level during the course of gestation, and these changes can vary from year to year. While we know that major changes in the level of energy and protein in the diet of cows in late gestation can affect the birth weight, vigour, and health status of the newborn calf, new research is finding that, contrary to the traditional view, cow nutrition in early gestation may have an impact on traits exhibited much later in the calf ’s life. And, in addition, dietary differences in late gestation which are not large enough to affect the neonatal calf may exert an influence much later in the animal’s life.
More from the Canadian Cattlemen website: Good cows don’t just happen
A Nebraska study looked at the effect of protein supplementation of grazing late-gestation cows on the performance of their heifer progeny. Heifer calf birth weights were the same for supplemented and unsupplemented groups, showing that the difference in nutrient supply was not great enough to have an impact on fetal growth. However, the heifer calves from supplemented dams had greater weaning weights, prebreeding weight, weight at pregnancy check and, most importantly, better pregnancy rate. This pregnancy rate difference was quite large, with heifers from non-supplemented dams at 80 per cent while heifers from supplemented dams were 93 per cent. And the heifers from supplemented dams had a 28 per cent advantage in the number of calves born in the first 21 days of the calving season.
All this occurred even though post-weaning gain and feed intake were the same for both groups of heifers. So in this study, a fetus developed in a uterine environment which benefited from additional protein exhibited a major economic advantage a year and a half after being born, although no differences were apparent when they were newborn calves!
A long-term study in Montana provided two diets classed as either marginal or adequate to pregnant cows. These cows were winter grazing (Dec.-Mar.) and supplemented with various harvested feeds. Heifer calves from these cows were then developed at two dietary levels during a 140-day period after weaning, giving a total of four management groups. The high-level diet was fed to appetite and resulted in a heifer growth rate of 1.5 lbs. per day. The low-level diet was fed at 80 per cent of appetite and gave a growth rate of 1.15 lbs. per day. In the winter after their first breeding season, each feeding group was maintained, with restricted heifers fed the marginal diet after they became cows and the full-feed heifers placed on the adequate winter diet. The performance of heifers through their first breeding season showed the expected effects of post-weaning diet on growth, carcass and reproductive performance (higher-growthrate heifers had superior performance), but there were no effects from the nutritional treatment the dams had been on.
However, female progeny from the dams fed the lower-quality diet had heavier body weights at five years of age, whether they were placed on the high or low level post-weaning and winter diet. Female progeny from dams on the high-quality diet, that were themselves fed the low-quality diet had the lightest body weight and lowest condition score at five years of age. And the females from the dams fed the lower-quality diet and then fed on the lower nutritional plane during development and the following winters had significantly lighter birth weight progeny than all other groups, although they also had the second-highest body weights at five years of age.
Something about being nutrient restricted as fetuses carried forward to their performance as adults, independent of how they were fed after weaning. And females from high-level dams that went on to a restricted diet had a lower body condition score at five years than all other dietary combinations.
What about feedlot performance from progeny of nutritionally restricted dams? When researchers from Ohio (Underwood et al.) put pregnant cows on native or improved pasture they found no difference in the birth weight of male calves but the steers from improved pasture dams had greater weaning weights, feedlot gain, carcass weight and fat level.
These and other results have stimulated scientists to take a closer look at the possible mechanisms behind fetal programming. Although 75 per cent of fetal growth occurs during the last two months of gestation, and we have generally focused attention on cow nutrition during this phase, the early nutrition of the fetus seems to be important in influencing performance far into the future.
This is likely associated with the early development of the placenta, especially the degree of blood vessel development. Although most of the fetal mass is deposited late in gestation, critical aspects of development such as the differentiation of cells and the start of organ formation are occurring at a much earlier time. This makes nutrition of the dam early in gestation much more important than previously thought, and runs contrary to our tradition of treating these early-gestation cows as having minimal nutrient requirements.
While our understanding of fetal programming in beef cattle is only at a rudimentary level, it looks like a promising area. Hopefully, we will eventually be able to recommend feeding strategies for pregnant beef cows which takes into account the effect of fetal programming on the future performance of both feedlot animals and females that are retained in the breeding herd.
Tom Hamilton is the program lead for beef production systems with the Ontario Ministry of Agriculture and Food.