Reproductive failure and reproductive losses cost the US beef and dairy industries over $1 billion dollars annually (Bellows et al., 2002). In addition, the number one reason for culling cows is pregnancy status (33%), closely followed by age and poor teeth (32%; NAHMS, 2008). Seeing these numbers, should make producers think twice about how they are working to improve reproductive performance in their cowherds in order to decrease the number of cows being culled for reproductive failure each year.

Today’s cow/calf producers have many different areas of focus depending on what the goals of their operations are. For many the focus is on feed costs, genetics and health; however, how many producers strictly select on reproduction? Even if the focus is nutrition, nutrients are partitioned based on priority with #1 - Maintenance; #2 - Growth; #3 - Lactation and #4 - Reproduction. Therefore the nutrient needs of the first three must be met before any nutrients get directed towards reproductive functions. This article will focus on the interactions of nutrition (specifically protein) and reproductive efficiency and touch on some current research pertaining to protein supplementation and its effects on reproduction.

During the winter season in the Upper Midwest, producers utilize stored forages to feed their cowherds. A common occurrence in dormant forages, such as lower quality grass hay and cornstalks, is protein deficiency; therefore protein supplementation is commonly utilized. A co-product of the ethanol industry, distillers grains (DG), are a common, economical protein supplement that many producers can pair with a low quality forage to make a nutrient rich diet that is cost effective. In addition, from 2007 to 2012 South Dakota has lost 15% of our pasture land (US Census of Agriculture 2007; 2012). Therefore, as producers lose pasture and explore alternative feeding systems, cornstalk/co-product diets are becoming more common and these diets are being fed for longer periods of time potentially into the breeding season. Research has shown that beef cows fed co-products (like DG) with low quality improved reproductive performance compared to non-protein supplemented females, and also improved performance and pregnancy rates in yearling heifers and cows.

However; some DG produced today may not have the same nutrient profile as the first “traditional” DG did. This is due to the result of current methods of removing oil that ethanol plants utilize in an effort to extract more value from the corn kernel. The resulting DG are more concentrated in protein, with less energy compared to traditional distillers. Therefore, when these “new generation” DG are used in the same way as traditional distillers, excess protein consumption may occur. Producers will need to pay attention to the nutrient profile and potentially re-evaluate their use in cow/calf systems.


Consistent scientific evidence from the dairy industry suggests that excess protein consumption is detrimental to reproductive efficiency. It has largely been recognized that concentrations of urea nitrogen (a product of protein metabolism) of 19 mg/dL and above in plasma, serum or milk, decrease conception and pregnancy rates. Postpartum interval also increases due to negative interactions between urea nitrogen, and the uterine environment and hormone production. However, contrary to the dairy literature, excess protein consumption has been reported to be much more favorable on the beef side.

Research from SDSU and Purdue shows that heifers fed to fulfill their protein requirements versus those fed excess protein above requirements (150%) was not detrimental to uterine environment, puberty attainment, interval or duration of estrus, pregnancy rate or oocyte quality (Gunn et al., 2014a; Amundson et al., In Press). In addition, the progeny from the heifers in the trial of Gunn et al. (2014a) were followed through their first gestation to parturition (Gunn et al., 2015a). The heifer progeny from excessive protein-supplemented dams were heavier from birth to puberty, offered more skeletal growth and had greater AI pregnancy percentage compared to the progeny from dams on the control treatment.

Furthermore, not only has excess protein shown generally not to negatively impact beef reproduction, but research focusing on supplementing an excess of specific types of protein has been reported to be beneficial to certain reproductive functions. Research from Kansas State and Iowa State looked at the effects of protein degradability and amount on reproductive performance. Soybean meal was used as the highly degradable protein source and corn gluten meal as the undegradable protein. Results indicated that excess undegradable protein increased milk production (Rusche et al., 1993) and follicle size (Geppert and Gunn, 2015). More work done by these researchers found increasing concentrations of undegradable protein was beneficial to ovarian follicle size (Gunn et al., 2014b; Geppert et al., In Press), which can be linked to greater pregnancy success (Perry et al., 2005; 2007). However, the more degradable supplements were found to increase hormone concentrations; therefore, additional research needs to be done to evaluate why these differences occur.

Lastly, a meta-analysis combining data from 15 studies, including more than 1,300 females tried to determine if there was a certain concentration of urea nitrogen (similar to dairy) that negatively affected pregnancy rate in beef cows. All females in these studies were acclimated to diet for 1 week prior to when samples were taken for analysis. Results indicated that first service conception rates and overall season pregnancy rates were not different from females that had in circulation 10 to 25 mg/dL of urea nitrogen. Furthermore, there was a tendency for a positive relationship between greater protein intake and pregnancy success (Gunn et al., 2015b).


In summary, it appears that beef cows are affected differently by excessive protein consumption than dairy cows. At this time there is no known threshold where excess protein intake (up to 25 mg/dL) negatively impacts reproductive performance of beef cows as long as adequate time for diet acclimation is allowed. Therefore, if producers utilize new generation distillers grains in the same way they used traditional DGs, reproductive efficiency should not be affected. Adequate attention should be placed on making sure nutrient requirements are met. At the present time, and based on reproductive research, it appears that protein requirements of today’s larger cows (1400 lb. vs. 1200 lb) could be higher, since more protein appears to be beneficial to reproductive success. More research is currently being conducted in this area and will hopefully shed some light on why these results are being seen.