Fetal Programming of Beef Cattle
Beef improvement programs have traditionally focused on genetic selection. Recent advances in genomics has accelerated selection by allowing us to identify key differences the DNA of an individual animal's DNA sequence. Wow! - being able to "look" right into an animals'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.
Genes and the Environment
The standard view of how an animal's performance is determined takes into account 2 main factors:
- its genetic makeup
- 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 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 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?
What is Fetal Programming?
Recent research shows that the future performance of an animal can be affected by the maternal environment of the fetus during its early development1. 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 (see Figure 1).
Figure 1: Schematic of the Principle of Fetal Programming
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 diet. 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 if 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.
A study from Nebraska2 looked at the effects of protein supplementation of grazing, late gestation cows on the performance of their heifer progeny. Heifer calf birth weight was the same for both 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% while that for heifers from supplemented dams were 93%. And the heifers from supplemented dams had a 28% advantage in the number of calves born in the first 21 days of the calving season. All this occurred even though postweaning gain and feed intake was 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 Montana3 provided two dietary levels (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 on 2 dietary levels during a 140 day period after weaning, giving a total of 4 management groups. The high level diet was fed to appetite and resulted in a heifer growth rate of 1.5 lbs/day. The low level diet was fed at 80% of appetite and gave a growth rate of 1.15 lbs/day. In the winters 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 postweaning diet level on growth, carcass and reproductive performance (higher growth rate heifers had superior performance), but there were no effects from the nutritional treatment their dams had been on.
However, measures taken later in life showed effects did relate to the maternal environment they developed in. Female progeny from the dams which were fed the lower quality diet had heavier body weights at 5 yrs of age, whether they were placed on the high or low level postweaning and winter diet. Female progeny from the dams which were fed the high quality diet, and were then placed on the low quality diet themselves had the lightest body weight and lowest condition score at 5 yrs 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 5 yrs of age. Something about being nutrient restricted as fetuses carried forward to their performance of adults, independent of how they were fed after weaning. . And females which were from high level dams and then went into the restricted dietary group had a lower body condition score at 5 yrs than all other dietary combinations.
What about feedlot performance of progeny from nutritionally restricted dams? Research from Ohio (Underwood et al. 4) with pregnant cows on either native or improved pasture found that although the birth weights of male calves were not different between groups, 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% of fetal growth occurs during the last 2 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 - contrary to our tradition of treating them as having minimal nutrient requirements.
While our understanding of fetal programming in beef cattle is at only 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.
1 Funston et al., 2012. J. Anim. Sci., 90 :2301-2307.
2 Martin et al. 2007. J. Anim. Sci. 85 :841-847
3 Roberts et al., 2009. J. Anim. Sci. 87 :3043-3042.
4 Underwood et al. 2010. Meat Sci. 86:588-593
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|Author:||Tom Hamilton - Program Lead-Beef Production Systems/OMAFRA|
|Creation Date:||30 October 2013|
|Last Reviewed:||30 October 2013|