|Effect of Nutrition on Reproduction in Dairy Cows|
|Written by Administrator|
|Wednesday, 16 March 2011 14:27|
Effects of Nutrition on Reproduction in Dairy Cows
A. Ahmadzadeh1, A. J. Young2 and D. Zobell2
1University of Idaho, 2Utah State University
(Paper presented on International Seminar of Dairy Cattle 2009 “Improving Productivity of Dairy Cattle and Dairy Product Using Natural Sources”
Faculty of Animal Science, Andalas University, West Sumatera Indonesia).
As milk production of dairy cows increases, postpartum nutrition plays a larger role in reproductive performance and thus the profitability of dairy herds. It appears that increased milk production has negatively affected conception rate. The increase in metabolic demands of high production along with demands on reproductive efficiency and cow health makes the interactions of postpartum nutrition and reproduction an important topic in the dairy industry. The mechanism by which nutrition affects reproduction is very complex and not clearly understood. However, the role of nutritional factors such as rumen degradable protein and energy balance and their mechanisms of action on reproduction has become more apparent in recent years.
To increase milk yield and overall profitability, dairy producers try to maximize feed intake, especially during the early postpartum period. Because high protein rations are generally more palatable and increase feed intake (10), dairy producers often feed crude protein in excess of requirements to cows during the this period. Feeding diets with high concentration of crude protein may reduce reproductive efficiency. In most studies, increasing dietary crude protein, has lowered fertility by increasing days to first postpartum ovulation, services per conception, and (or) days open. However, some researchers question the effects of feeding excess crude protein on fertility. The inconsistencies observed in the literature may be due to the source of dietary protein rather than total dietary crude protein. Ferguson and Chalupa (6) believe that increasing dietary crude protein will not necessarily be associated with lower conception rate, and furthermore, total dietary crude protein does not adequately describe its interaction with reproduction. Instead, supply and requirement for rumen degradable and undegradable protein may better describe the interaction between reproduction and protein intake. Feed stuffs differ in the degradability of their protein fraction. For instance, two rations containing 18 % protein may differ in amount of ruminal degradable and undegradable protein. Changing crude protein intake on a dry matter basis may provide excesses or deficiencies of rumen undegradable protein and rumen degradable protein, which may result in an imbalance in rumen utilization of nitrogen and amino acids absorbed from the small intestine (6).
Anderson and Barton (1) suggested that elevation of tissue ammonia associated with high intakes of crude protein may delay clearance of uterine contaminants by reducing immune system function. In herds which experience reproductive problems with early postpartum cows, feeding a high crude protein diet containing high rumen degradable protein, may decrease the fertility, unless the herd health program provides for early identification and treatment of problem cows. Three general theories have been proposed to describe how excess dietary protein may negatively influence fertility:
1) Toxic by-products of nitrogen metabolism from the rumen (ammonia) and liver (urea) may impair sperm, ova, or early embryo survival;
2) Imbalances in protein and energy supply may affect efficiency of metabolism;
3) Nitrogen by-products or efficiency of energy utilization may alter gonadotropin and (or) progesterone secretion. These effects may occur singularly, simultaneously, or synergistically. Progesterone is important for follicular development, passage of the fertilized embryo through the oviduct to the uterus, and maintaining pregnancy. The amount and source of protein may influence progesterone (10). It is possible that decreases in serum progesterone occur at concentrations of dietary crude protein that exceed rumen requirement for rumen degradable protein. However, source of protein, energy status and age of the animal should be considered. Nonetheless, the effect of protein consumption on progesterone concentration needs more study, and factors such as total energy intake and protein source should be examined.
The dietary protein in cattle rations is hydrolyzed to peptides and amino acids by ruminal bacteria. Amino acids then are degraded further to organic acids, carbon dioxide, and ammonia. Ammonia can be used by the rumen bacteria to synthesize protein. However, due to the level and source of dietary protein and the level of soluble carbohydrates, the release of ammonia is often too rapid for bacteria to utilize all available ammonia, especially if there is not enough energy for the bacteria to utilize the ammonia (10). As a result, excess ammonia will move through the wall of the rumen and enter the liver via the bloodstream and is converted from the toxic ammonia to the nontoxic urea. This is either excreted in urine or recycled back to the rumen. A high level of ammonia in the rumen will cause an increase in rumen pH and increases the absorption rate of ammonia from the rumen (10). Subsequently, blood urea nitrogen (BUN) and milk urea nitrogen (MUN) levels will increase. The degradability of the dietary protein fraction varies and may differ from 100% for urea to about 20% for blood meal. As was mentioned, BUN is influenced not only by crude protein intake, but also by degradability of protein. Excess crude protein (above the requirements) with a high rumen degradable protein content results in an increase in ruminal ammonia and thus a higher urea concentration in body fluid and BUN.
Role of Energy Balance on Reproduction
During the early postpartum period high producing dairy cows are unable to consume enough feed to support energy demands for milk production. When nutrient intake does not meet increased dietary needs of milk production, negative energy balance occurs. In this situation, the energy requirements are met partially through the mobilization of body reserves which results in loss of body weight and body condition. Excessive mobilization of body reserves has been associated with postpartum fat infiltration of the liver and reduced reproductive performance in high yielding dairy cows (3). At least 80 % of dairy cows experience negative energy balance during early lactation and its magnitude and duration depend more on feed intake than milk yield. Deficiencies in energy intake probably will become more common as milk yields increase in dairy herds.
The mechanisms by which insufficient postpartum feed intake and thus negative energy balance affect reproduction is not clearly understood. However, there are several potential pathways which may explain this antagonistic relationship between postpartum metabolism and reproductive performance. Luteinizing hormone (LH) is a critical hormone which is required for re-establishment of ovarian activity, final growth and maturation of the ovulatory follicle, ovulation, and ovarian secretion of progesterone. An extremely negative energy balance may alter secretion of LH and as a result affect follicular development and delay ovulation (4). According to Nebel and McGilliard (9), timing and magnitude of negative energy balance affect luteinizing hormone secretion and , therefore, secretion of progesterone, which affects expression of heat and support of the uterus during the early pregnancy.
Early postpartum negative energy balance may cause low fertility by negatively affecting the quality of follicles destined to ovulate during the breeding period. It takes about 70 days for a follicle to become fully mature and ready to ovulate. Follicles exposed to adverse conditions, such as severe negative energy balance in the early postpartum period may not be functional 70 days later when the producer is ready to breed the cow. That, by itself, may indicate how critical it is to maximize feed intake as early as possible to increase the energy balance.
The severity of postpartum negative energy balance and the delay in the initiation of normal postpartum reproductive cyclicity is associated with body weight and body condition loss. The existence of such a relationship would make body condition scoring a useful management tool for relating the reproductive performance of the herd to its nutrition status. It has been shown that high producing Holstein cows that lost body condition dramatically during the first 5 weeks postpartum, had significantly longer intervals to first ovulation, significantly lower first service conception rate, and over all lower conception rates (Table 3).
Higher conception rates in lactating cows appear to be related increased number of estrous cycles prior to breeding. Therefore, feeding strategies that hasten initiation of postpartum ovulatory cycles may have beneficial effect on reproductive performance (5).
It is evident that the effects of nutrition and energy status on reproductive performance of dairy cattle is one of the great challenges which both nutritionists and physiologists face. If the energy balance status and level of protein in cattle diets influence reproductive functions, then manipulation of the metabolic state by means of nutrition and other management practices should improve the reproductive efficiency of dairy cattle. A re-evaluation of the ration may suggest balancing for rumen degradable and undegradable protein rather than total crude protein in the ration. Additionally, body condition scoring may provide a practical tool for monitoring the energy status and thus reproductive performance (9) . Obviously, cows need to be healthy and should be provided with fresh palatable feed so that feed intake can be maximized. However, the feeding strategies of maximizing intake in high producing cows, so that severity and length of negative energy balance can be minimized, requires more research. As Swanson (10) mentioned, "more graduate students need training in the area of the interrelationship of nutrition and fertility to study the integrated effects of nutrition and fertility."
1) Anderson, G. W., and B. Barton. 1987. Reproductive efficiency:potential nutrition management interactions. New England Feed Dealers Conf., Univ. Maine. Orono.
2) Britt, J. H. 1992. Impact of early postpartum metabolism on follicular development and fertility. New concepts in the interactions of nutrition and reproduction. Bovine proceedings.39.
3) Butler, W. R. and R. D. Smith. 1989. Interrelationship between energy balance and postpartum reproductive function in dairy cattle. J. Dairy Sci. 72:767.
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8) Harris, B. 1995. Using MUN and BUN values as management tools. Udder information. 1(4).
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13) Visek, W. J. 1984. Ammonia:its effects on biological systems, metabolic hormones and reproduction. J. Dairy Sci. 67:481.
14) Johnson, R.G. and A.J. Young. 2003. The association between milk urea nitrogen and DHI production variables in Western commercial dairy herds. J. Dairy Sci. 89:3008-3015.
|Last Updated on Wednesday, 16 March 2011 14:37|