Effect of Dietary Protein and Fat on Fertility

During early lactation, high producing cows do not consume enough feed to meet the demands for milk production. This leads to a state of negative energy balance which, when prolonged, adversely affects the reproductive performance of the animal. One way of overcoming this energy deficit is to increase the energy density of the diet with supplemental fat. Excessive levels of protein fed in an effort to maximize milk production may also impair reproduction.

How does protein affect fertility?

Most studies have demonstrated that an excess of rumen degradable protein (RDP) in the diet has a deleterious effect on fertility. High rumen ammonia levels, resulting from excess RDP, have been associated with an increase in plasma urea nitrogen (PUN) and a decrease in pH of the uterine environment. Table 1 shows first service conception rates, estrous cycle length and PUN levels in heifers fed either normal (CP-15%; Urea-0.5%) or high (CP-21%; Urea-3.2%) levels of RDP.

Parameter	Protein Level
	Normal	High
1st service conception rates	82%	61%
Av. estrous cycle length, days	21	24
Peak PUN (mg/dl)	17-18	23-24

Table 1 : Effect of a normal or high protein diet on some reproduction parameters and plasma urea nitrogen (PUN).
Source: Elrod, C.C. 1993. J. Anim. Sci. 71:694-701.

The adverse effects of high RDP in the diet can be avoided by supplementing with rumen undegradable protein (RUP). Since RUP has a stimulatory effect on milk production and therefore on the magnitude of energy deficit, high RUP diets without adequate energy intake may also have a deleterious effect on reproduction. In fact, high RUP diets were found to prolong the interval to first estrus postpartum. Low protein intakes have been associated with decreased occurrence of estrous and reduced conception rates.

How does fat improve fertility?

Most studies have shown that increasing the fat content above 3% of the diet DM, in the early postpartum period, helps in improving milk yield and pregnancy rates. In some studies, supple-mental fat improved body weight (BW) or body condition score (BCS), with a corresponding increase in conception rates. In other studies, conception rates improved even though BW/BCS decreased. This suggests that dietary fat exerts a positive effect on reproductive performance by means other than its effect on the energy status of the animal.

During the early postpartum period, the uterus secretes large quantities of prostaglandin F2-alpha (PGF2a), a hormone which causes the uterus to shrink back to its normal non-pregnant size. In cycling cattle, PGF2a plays an important role in the regression of the corpus luteum (CL) present on the ovary. However, when the cow conceives, the embryo sends signals to shut down PGF2a secretion from the uterus so that the CL remains active. The active CL secretes another hormone, progesterone, which establishes a favourable uterine environment for embryo survival and pregnancy.

Recent studies show that supplemental fat improves fertility by stimulating progesterone secretion and/or by inhibiting PGF2a release. Dietary fat was found to increase plasma cholesterol (a precursor for progesterone) and progesterone concentrations, with a concomitant reduction in estradiol levels. Further, a low progesterone clearance rate also contributes to elevated progesterone concentrations. Fat is also suspected to stimulate the secretion of luteinizing hormone (LH). All these factors contribute to an increase in the number and size of CL and enhance embryo survival.

What kind of fat improves fertility?

The source and type of dietary fat has a significant influence on reproduction. For example, fish meal was found to increase pregnancy rates in most studies. Some of the polyunsaturated fatty acids (PUFA) in fish meal which bypass the rumen include linoleic acid , eicosa-pentaenoic acid and docosahexanoic acid. Since these fatty acids inhibit the synthesis of PGF2a by the uterus, the lifespan of the CL is prolonged and early embryonic death could be prevented. Feeding fish meal at as low as 2.8% of diet DM has improved conception rates of lactating cows. Studies conducted in the U.S., Israel and Ireland indicate that fish meal is particularly beneficial when:

Apart from fish meal, feeding of ruminally inert fats like CaLCFA (Calcium salts of long chain fatty acids: Megalac®) also resulted in an increase in the number and size of CL on the ovary. In most studies, feeding CaLCFA (Megalac®) at 1 kg/day was found to increase conception and pregnancy rates. Though CaLCFA and fish meal are expensive, these feeds should be cost-effective when used in situations where conception rates are low and culling rates are high. The data from some research studies, on the effects of supplemental fat on reproductive performance, are summarized in table 2.

Treatments	Cows per treatment	Milk yield (kg/day)	Days open	AI per conception	Pregnancy rate (%)
0% inert fat	21	24.8	76	1.35	52
2.2% inert fat	22	26.4	84	1.45	86
0% inert fat	223	+1 (heifers)	138	1.74	85
450 g/day inert fat	220	+1.5 (cows)	146	1.71	79
0% tallow	34	30.7	88	1.36	44
3% tallow	34	31.4	95	1.25	62
0% fish meal	146	46.1	74	1.4	32
2.8% fish meal	154	46.2	77	1.4	41
0% fish meal	31	32.7	87	1.2	84
3.5% fish meal	31	33.8	82	1.4	86

Table 2 : Effects of supplemental fat on reproductive performance of lactating dairy cows. Selected data from Staples et al. 1998. J. Dairy Sci. 81:856-871.

It is not clear whether whole oil seeds (cottonseed, soybean or sunflower) are effective in increasing the delivery of PUFA postruminally and in improving pregnancy rates. In some studies, intestinal delivery of PUFA was improved by supplementing ionophores with fats, by treating soybean oil with butylamine, or by treating whole canola seed with alkaline peroxide. Protection of dehulled cottonseed and linseed with protein-aldehyde complexes (protected lipid) was also found to improve pregnancy rates from 63% to 79%.

Conclusions 

Extremes in the supply of dietary nutrients, especially protein, have deleterious effects on postpartum conception rates. Though the adverse effects of excess RDP on fertility have been well documented, the beneficial effects of RUP on conception rates are not yet clear. In contrast, dietary fatty acids, especially from fish meal, have consistently improved pregnancy rates. Fat supplementation should be especially useful in situations where conception rates are low.