Dietary crude protein (CP) is an important determinant of milk production. Underfeeding CP is associated with reduced peak milk production. The partitioning of dietary CP into rumen degradable protein (RDP) and rumen undegradable protein (RUP) fractions has enabled a better understanding of protein utilization in the dairy cow. It has been recognized that feeding balances of RDP and RUP consistent with requirements for rumen microbial synthesis and milk production can improve nitrogen (N) utilization efficiency. In addition to improving milk production, providing sufficient balances of RDP and RUP may enhance fertility and reduce environmental losses of N. Protein and fertility In general, increasing CP in dairy rations has been associated with reduced fertility, measured by increases in services per conception (reduction in conception rate (CR)) or days open (a measure of reproductive efficiency). However, when studies were combined in a meta-analysis, CP had no association with CR, but excess of RDP above that needed for rumen microbial synthesis was associated with reduction in CR. Rumen requirement for RDP is largely determined by fermentable carbohydrate. NRC estimates the requirement of RDP as 1.18 times the yield of microbial crude protein, which is assumed to be 130 grams per kilogram (g/kg) total digestible nutrients (TDN) intake. This would equate to 24.5 g of degradable N per kg of TDN. Supplies of RDP providing more N per kg of TDN would increase rumen ammonia, plasma urea nitrogen (PUN) and milk urea nitrogen (MUN) concentrations. MUN and PUN are highly correlated; relative differences in MUN and PUN concentrations will depend upon time of sampling PUN relative to feeding and sampling of MUN from composite or a.m.-p.m. milk samples. MUN sampled from composite a.m.-p.m. milk samples tends to be a more stable estimate of MUN concentrations. Plasma and MUN will be used interchangeably in this [article]. Ferguson et al. observed that fertility in a dairy herd was sensitive to elevated PUN. During periods when diets were offered with elevations in RDP, which increased PUN, CR declined in the herd. Cows with PUN greater than 20 milligrams per deciliter (mg/dl) had CR under 25 percent. The data suggested that PUN concentrations above 20.8 mg/dl were detrimental to fertility. Canfield et al. associated elevated PUN with reduced CR in an experiment with higher dietary RDP. Several studies have examined increasing PUN or MUN and CR in dairy cows. A likelihood ratio test (LRT) for pregnancy was calculated for several of these studies. The LRT is calculated as the proportion of pregnant cows divided by the proportion of open cows within each urea category, as a proportion of the total cows. Pregnancy is more likely when the LRT is greater than one and less likely when below one. In general, as urea concentration increases in plasma or in milk, fertility declines. However, the decline in fertility is not uniform across the studies, and the highest fertility group in Godden et al. was in the highest MUN category. This suggests that there is a general trend in reduction in fertility with increasing MUN, but MUN alone does not predict fertility. Multiple factors influence fertility. Other risk factors for fertility, not identified in these studies, may modify the association of increasing urea on fertility in dairy cows. Westwood et al. found that cows consuming increased RDP had lower fertility when associated with greater weight loss in the early postpartum period, suggesting energy balance may play a modifying role on nitrogen effects on fertility. Other factors may include body condition loss, metritis and earlier days of first insemination. Melendez et al. found negative associations of increasing MUN with fertility in summer versus winter months. Cows may adapt to high urea levels and maintain fertility. Godden et al. found the relationship of fertility with urea was quadratic; fertility was higher in cows with low and high MUN concentrations. Gustafsson et al. observed a similar relationship in Swedish herds. Increased MUN is correlated with increased urinary urea. Urinary urea breaks down rapidly to ammonia when mixed with feces. Ammonia volatilizes rapidly from barn floors and contributes to air particulate matter and acid rain. Therefore, reducing MUN has other benefits[besides] reproduction. Together, the results suggest that fertility and environmental impact (and milk production) may be minimized when MUN concentrations are maintained between 9 to 16 mg/dl on a herd basis. Individual cow concentrations may range from 4 to 22 mg/dl, but the majority of animals will cluster between 9 to 16 mg/dl. Thus, high production can be supported with adequate protein and minimal urea concentrations. Mechanisms reducing fertility Specific actions by which increasing urea concentrations associated with excess RDP reduce fertility have not been identified. Effects may be associated with alterations in the uterine environment which are detrimental to the early embryo or effects may be detrimental to the oocyte, retarding development of the early blastocyst. Blanchard et al. observed embryo quality was reduced in cows consuming a 16.5 percent CP diet that contained 70 percent RDP compared with 62 percent RDP. The effect was not apparent in all cows, but particularly was seen in a higher proportion of cows in their 4th or greater parity. Approximately one-third of cows consuming the higher RDP diet failed to yield any fertilized embryos. Larson et al. found that cows with higher MUN had more failed pregnancies, which were associated with regular inter-estrous intervals, based on sequential milk progesterone testing. These data suggest higher RDP and urea concentrations are associated with fertilization failure as a cause of repeat breeding and should result in regular inter-estrous intervals. However, Elrod et al. observed that reduced fertility with increasing serum urea nitrogen in heifers was associated with increased inter-estrous interval and reduction in uterine pH early in the luteal phase. Infertility was associated with increased embryonic loss. Elrod’s work suggested that loss of embryos occurred after maternal recognition of pregnancy, which extended the inter-estrous interval, resulting in reduced fertility. These results are in contrast to Blanchard et al. and Larson et al. Blanchard and Larson’s studies were in lactating dairy cows, whereas Elrod’s studies were in primiparous, nonlactating cows. Therefore, mechanisms may be different. In addition, Blanchard’s study involved embryo’s collected from super-ovulated cows, seven days post-insemination, whereas Larson’s data was based on progesterone profiles post-insemination. Embryo loss prior to day 15 may have resulted in normal inter-estrous intervals in Larson’s study. Sinclair et al. found higher dietary RDP increased serum ammonia and effected oocyte maturation and early blastocyst development. McEvoy et al. observed that plasma ammonia concentrations measured at or near insemination in sheep were negatively correlated with pregnancy. These studies suggest that increases in serum ammonia may play a role in reducing reproductive performance in cows fed high RDP diets by influencing oocyte quality and blastocyst maturation. DeWit et al. and Ocon and Hansen found that oocytes incubated in increasing concentrations of urea had reduced proportions of fertilized oocytes that developed to blastocysts. DeWit et al. found that increasing urea was associated with reduced fertilization and cleavage rate, but had no effect on embryos after fertilization. Ocon and Hansen reported that fewer oocytes developed to blastocysts due to decreased developmental competence. Urea reduced fertilization and cleavage rate of developing embryos. Armstrong et al. found increased urea associated with increased nutrient supply decreased oocyte quality. However, Lavan et al. observed that Holstein cows fed diets high in rapidly rumen degradable nitrogen experienced no negative effects on follicular development or embryo growth despite increases in serum urea and ammonia, suggesting cows can adapt to short-term increases in RDP. Few studies have examined the relationship between RUP and fertility. Westwood et al. concluded that increasing RUP in isonitrogenous diets, improved feed intake, reduced serum nonesterified fatty acids postpartum and improved reproductive performance particularly in cows of high genetic merit. Triplett et al. fed a basal diet to postpartum beef cows with three supplements of increasing RUP (low RUP, 38.1 percent; moderate RUP, 56.3 percent and high RUP, 75.6 percent). Cows receiving the low RUP supplement had lower first-service CR than cows receiving the moderate and high RUP supplement (29.2 percent versus 57.6 percent and 54.6 percent, respectively). Overall pregnancy proportion tended to be lower for the cows receiving the low RUP supplement than the moderate and high supplements (43.2 percent, 61.5 percent and 56.4 percent, respectively). It is difficult to separate the effects of increasing RUP on fertility from the simultaneous reduction in RDP which occurred in these studies. Conclusion Risk factors which modify N effects on fertility have not been clearly identified. Although it seems fertility may be maintained at higher MUN concentrations, the general trend across the literature is a reduction in fertility. In addition, elevations in MUN are associated with increased urinary losses of N, a form of N which will be rapidly lost as ammonia to the environment. Nutritionists and veterinarians can monitor milk urea nitrogen (MUN) as a tool to assess efficiency of protein feeding. Mean MUN between 9.0 to 14 mg/dl is sufficient for adequate milk production and will ensure there are no negative effects on reproduction. Concentrations of MUN between 14 to 16 mg/dl should not significantly impair fertility but indicate some wastage of dietary N is occurring. MUN concentrations above 16 mg/dl not only may decrease fertility but also increase the risk of environmental pollution from ammonia volatilization. PD References omitted but are available upon request at editor@progressivedairy.com —From 2007 Mid-Atlantic Nutrition Conference Proceedings