Dr. Benjamin Wenner, Assistant Professor, Department of Animal Sciences, The Ohio State University
As we approach the winter lambing season in Ohio, producers have a variety of approaches to feeding pregnant ewes. Those who believe underfeeding their ewe will decrease fetal size are partially correct (as addressed in the ASIA Sheep Production Handbook, 2002), but the likelihood of decreasing dystocia with underfeeding is nearly nil. In a 2007 review of lambing data, late gestation energy supplementation could account for increasing fetal weight by roughly ½ lb. (Gardner et al., 2007). Certainly, there are many other factors leading to dystocia that deserve consideration before a ½ lb. increase in lamb birth weight garners attention. Twinning alone can reduce birth weights (despite increasing ewe conceptus weight and energy requirement) and thus practices to achieve greater fertility in your breeding flock are a wiser pursuit than trying to nutritionally limit birth weights during gestation.
It has become common practice to put ewes on marginal feeding once pregnant. In fact, 30 days into gestation, the fetus only increases the ewe’s demand for energy or protein by roughly 2%. Of course, like many species, the pregnant ewe will cry for feed and make you feel guilty but free-choice hay alone can often exceed her requirements in the first 2-3 months of gestation depending on the quality provided. Some exceptions apply, especially that the colder temperatures may increase sheep requirements by 10-15% depending on the temperature, precipitation, wool cover, and shelter. But the general consensus is that early- and mid-gestation ewes have minimal increases in requirements compared to their maintenance diet. So when do we need to start supplementing these ewes and by how much?
The increased demand of the fetus(es) for energy and protein both grow dramatically during the last 6 weeks of gestation. Academically, we’d estimate these increased requirements in metabolizable energy and protein – estimated as the amount of energy or protein that is digested, absorbed, and available to the animal for metabolic processes. By the end of 4 months of gestation, the average 200 lb. ewe, bearing twins, has an increased requirement for both metabolizable energy (ME) and metabolizable protein (MP) by nearly 60%. One month later, she has reached a 120% increase in ME and MP requirements by lambing! This dramatic increase in both ME and MP requirements at the end of gestation is captured graphically for a variety of lamb and birth weight combinations. And it is these last 2 months of gestation where the average quality grass hay can no longer provide the nutrients demanded by the developing lambs.
This leads to the question of titles. If both the ME and MP requirements are increasing dramatically in tandem for the last 6 weeks of gestation, why does the title lead you to believe you should focus on the energy supplementation? As a general rule across sheep production systems, sheep are commonly fed in excess of their requirements for protein. Overfeeding crude protein (CP) is certainly better than the alternative of shorting your sheep on the essential building blocks for all measures of farm productivity (meat, milk, wool). However, when evaluating the diets balanced for sheep versus their listed requirements in the NRC for small ruminants (2007), diets can accidentally exceed CP requirements by up to 20% quite often. Rations for show stock reach a fresh level of excess CP as we aim to capture every ounce of muscle gain possible, regardless of cost. But that is a topic for a different day.
By no means am I implying that protein is an innocent bystander to the conversation on ewe supplementation. Early in gestation, there is evidence that amino acid supplementation of the ewe can influence the genetic expression of her offspring (Sinclair et al., 2007) and that underproviding CP to ewes can influence brain pathways related to energy usage in the lamb (Begum et al., 2012). However, the greatest risk of protein deficiency is in a mishandled flushing scenario (high provided energy decreases pasture/hay intake and dietary CP) or in late gestation when ewe intake and rumen volume are depressed by fetal encroachment.
However, it is much more likely that a pregnant ewe will be deficient in ME than MP during late gestation. Depicted in the figure is the net balance of energy without supplementation by month (blue bars) versus the net balance of energy with supplementation of corn in months 4 and 5 of gestation (orange bars). As the ewe loses gut volume for hay intake, the increased need for supplemental energy could require 1-1.5 lbs./d in month 4 and ultimately reach 2-3 lbs./d whole corn by the time of delivery. This corn provides marginal quantities of MP but will also drive increased microbial growth in the rumen, supplying additional MP to the ewe. Thus, it is likely with corn supplementation in an average ewe (200 lbs., 10-12 lb. twins) that MP requirements of the ewe will be met incidentally.
The two primary categories of nutrient-induced pre- and post-parturition diseases revolve around energy and calcium. Making sure the ewes have access to a free-choice mineral can help prevent hypocalcemia, making energy your primary concern. As you move into the winter months and prepare for lambing, the best way to monitor the need for supplementation remains to body condition score your ewes. Putting a hand on them at the feed bunk or through the chute is still the simplest way to monitor change in condition over time. Ewes should not noticeably gain or lose BCS through late gestation and if you sense a shift in the group average then it’s best to adjust supplementation immediately. Based on the size of ewe and number of lambs, supplementing 1-2 lbs. of corn up to 3 lbs. of corn at the end of gestation can help prevent many issues around parturition and improve the viability of your lamb crop.