Impact of protein nutrition on reproduction in dairy cows

Review: effect of protein nutrition on ovarian and uterine physiology in dairy cattle

Milk production and dry matter intake of dairy cows are stimulated in response to increased intake of dietary protein, but, unfortunately, decreased fertility is often associated with this nutritional strategy. Ruminally degradable protein or ruminally undegradable protein in excess of requirement can contribute to reduced fertility in lactating cows. Dietary protein nutrition or utilization and the associated effects on ovarian or uterine physiology have been monitored with urea nitrogen in plasma or milk; concentrations above 19 mg/dl have been associated with altered uterine pH and reduced fertility in dairy cows. The uterine pH changed dynamically and inversely with plasma urea nitrogen, signaling possible changes in the uterine milieu. Mechanisms for reduced fertility include exacerbation of negative energy balance and reduced plasma progesterone concentrations when cows were fed rations that were high in ruminally degradable intake protein. Alternatively, changes in uterine secretions that are associated with high protein intake and elevated plasma urea nitrogen might be detrimental to embryos. Bovine endometrial cells in culture respond directly to increasing urea concentrations with alteration in pH gradient but respond most notably with increased secretion of prostaglandin F2 alpha (PGF2 alpha). Increased uterine luminal PGF2 alpha interferes with embryo development and survival in cows, thus providing a plausible link between elevated plasma urea nitrogen concentrations and decreased fertility. Poor fertility in high producing dairy cows reflects the combined effects of a uterine environment that is dependent on progesterone and rendered suboptimum by the antecedent effects of negative energy balance or postpartum health problems and that is further compromised by the effects of urea resulting from intake of high dietary protein.

Indications and implications for testing of milk urea in dairy cattle: A quantitative review. Part 2. Effect of dietary protein on reproductive performance

DIETARY PROTEIN AND DAIRY COW FERTILITY: Feeding more dietary protein has been negatively associated with dairy cow fertility in some but not all studies. We used meta-analysis to examine the relationship between dietary crude protein and conception rate. While a higher intake of dietary crude protein significantly lowered conception rate, the potential for feeding less degradable dietary protein to modify this relationship was not demonstrated. MILK UREA CONCENTRATIONS AND DAIRY COW FERTILITY: The use of milk urea as an indicator of dietary energy and protein intake and as an indicator of reproductive performance has been questioned. We found that changes in urea concentration in body fluids explained only 25% (p = 0.08) of the variance in conception rate after conducting a meta-analysis of available studies. INTERPRETATION OF MILK UREA CONCENTRATIONS: High intakes of dietary protein may induce adaptations in urea metabolism, and the negative relationship identified between high intakes of dietary protein and fertility for Northern Hemisphere dairy herds may not necessarily apply in Australasian dairy herds. Because of the potential for cows to adapt to high protein diets, the use of a single milk urea determination on a herd will have limited value as an indicator of nutritional status and little value as a predictor of fertility.

Protein degradability and calcium salts of long-chain fatty acids in the diets of lactating dairy cows: productive responses

Our objective was to evaluate the effect of excessive intake of ruminally degradable crude protein [11.1 and 15.7% of dietary dry matter (DM)] and supplemental fat (Ca salts of long-chain fatty acids at 0 or 2.2% of dietary DM) on the productive performance of lactating Holstein cows (n = 45) during the first 120 d postpartum. The main N sources were soybean meal and urea in the diets with high concentrations of degradable protein versus a combination of vegetable and animal by-product feedstuffs in the diets with less degradable protein. Cows fed the diets with excess degradable protein had slower rates of increase in DM intake (DMI) and milk production, had lower plasma insulin and greater plasma glucose and urea concentrations, and lost more than twice the body weight of cows fed the diets with less degradable protein. Supplemental fat in the highly degradable protein diet reduced the loss of body condition, stimulated DMI, and reduced concentrations of plasma nonesterified fatty acids early postpartum compared with the highly degradable protein diet without added fat. Without affecting DMI, supplemental fat stimulated milk production (2 kg/d) starting at 3 wk postpartum. During early lactation, DMI and milk production were sensitive to the degree of ruminal degradability of protein and energy supplementation in the form of fat.

Protein degradability and calcium salts of long-chain fatty acids in the diets of lactating dairy cows: reproductive responses

Multiparous Holstein cows (n = 45) were assigned at calving to one of four diets arranged in a 2 x 2 factorial design. The two main factors were dietary concentration (dry matter basis) of 1) degradable intake protein (11.1 or 15.7%) and 2) supplemental fat (Ca salts of long-chain fatty acids; 0 or 2.2%). Soybean meal and urea were replaced with less degradable protein meals (corn gluten meal, meat and bone meal, fish meal, and blood meal). During the first 9 wk postpartum, cows fed diets containing the greater concentration of highly degradable protein demonstrated less follicular development on their ovaries, were delayed in their first luteal activity postpartum (25.2 vs. 38.6 d), accumulated less luteal tissue (< 15 vs. > 70 mm), and had lower plasma progesterone accumulated over time. The supplementation of Ca salts of long-chain fatty acids to the 15.7% degradable protein diet doubled the number of corpora lutea, reduced time to first rise in progesterone by 6 d, doubled the number of normal luteal phases, and restored the pattern of accumulated plasma progesterone concentrations to a pattern that was similar to that induced by other diets. Cows were synchronized to estrus and inseminated at approximately 65 d postpartum. Pregnancy rate was increased from 52.3 to 86.4% when fat was supplemented. Cows fed fat tended to have more corpora lutea and a larger corpus luteum and accumulated more plasma progesterone than did cows not fed fat. Diets containing excess degradable protein or Ca salts of long-chain fatty acids influenced ovarian structures and reproductive performance.

A veterinary perspective of on-farm evaluation of nutrition and reproduction

Reproductive herd health programs of the 1960s and 1970s focused primarily on the control of infectious and noninfectious diseases of the reproductive tract, estrus detection, breeding technique, semen quality and handling, and endocrine imbalances. Despite veterinary intervention, conception rates dropped from 55% before this period to 45% after this period, but milk production increased greatly. Because studies have shown that the additional nutritional needs for high producing cows may be a factor limiting reproduction, bovine practitioners have become more involved with dairy nutrition. New emphasis has been placed on the concept of total production medicine with the anticipation that improved nutrition would improve reproduction. Because of a better understanding of the dynamics of energy in the dairy cow and its effect on reproduction, fertility examinations now include nutrition monitoring and body condition scoring. Several relationships between nutrition and fertility have been identified: high producing thin cows that drop 0.75 to 1.0 in body condition score resulting in anestrus; fat dry cows that develop fatty livers and associated postpartum disease; heifers that have good milk production but prolonged anestrus; dry cows with ration imbalances during the transition period that develop milk fever, retained placenta, displaced abomasum, metritis, or endometritis; lactating cows with disease in midlactation, especially feet and leg problems, resulting in lowered fertility; and healthy, lactating cows with poor conception that have high concentrations of urea N in blood or milk.

Effect of dietary energy and protein concentration on the concentration of milk urea nitrogen in dairy ewes

To study the effects of dietary crude protein (CP) and energy on milk urea N concentrations in dairy sheep, eight pelleted total mixed rations were prepared to obtain two levels of energy density (1.65 and 1.55 Mcal of net energy for lactation per kilogram of dry matter for high energy and low energy rations, respectively) and four concentrations of CP within each energy level (mean CP concentrations, 14.0, 16.4, 18.7, and 21.2% of dry matter). The experimental design consisted of two 4 x 4 Latin squares (one per energy level) with two replications per treatment within each 3-wk period. Milk urea N concentrations were similar between dietary energy levels. Within each energy level, milk urea N was linearly and positively associated with dietary CP content and intake (range of milk urea N concentrations, 12.2 to 25.8 mg/dl for ewes fed high energy rations and 12.9 to 26.7 mg/dl for ewes fed low energy rations). The comparison of these results with those from other trials suggested that milk and blood urea N concentrations are closely correlated with dietary CP concentrations and less closely correlated with dietary CP intake. Our results suggest that milk or blood urea N concentrations can be used as indicators of protein metabolism and intake of lactating ewes.

Seasonal fluctuations in plasma concentrations of luteinizing hormone and progesterone in Brahman (Bos indicus) and Hereford-Shorthorn (Bos taurus) cows grazing pastures at two stocking rates in a subtropical environment

The effects of day length and grazing intensity on seasonal fluctuations in plasma concentrations of luteinizing hormone (LH) and cyclic ovarian activity were determined in Brahman (Bos indicus) and Hereford-Shorthorn (Bos taurus) cows maintained at two stocking rates in a subtropical environment. Contemporary groups of ovariectomised cows were monitored for fluctuations in plasma concentrations of LH. Equal numbers (n = 5) of entire and ovariectomised Brahman and Hereford-Shorthorn cows were assigned to a pasture with a greater or lesser stocking rate. Over a 15-month period, live weight was recorded weekly, and a blood sample was taken at the same time for measurement of plasma LH in entire and ovariectomised cows, and plasma progesterone in entire cows. Plasma concentrations of progesterone were used as an index of cyclic luteal function (time of cessation or onset of oestrous cycles). Regression coefficients were calculated to determine the least-order regression coefficient (LORC; range 1st to 10 order) for which time of year explained at least 50% (r2 > 0.05) of changes in live weight, plasma LH, or plasma progesterone; regression coefficients of 4th and 5th order indicated seasonally-related changes in these variables. For all cows, live weight was greatest in late summer to early autumn and lowest in winter. Changes in live weight were more closely related to seasonal changes in pasture availability for cows on pastures at a greater stocking rate (LORC 4th-5th) than for cows on pastures at a lesser stocking rate (LORC 1st-3rd). Cyclic ovarian activity ceased in four Hereford-Shorthorn cows on pastures at a greater stocking rate in late autumn to early winter, and onset of oestrous cycles did not occur in all of these cows until late spring. Oestrous cycles were not detected in one of five cows in the other groups during different periods of the study; however, there were no apparent patterns to cessation of oestrous cycles in these groups. There were no seasonally-related changes in plasma LH in entire cows, at either stocking rate (LORC 10th; r2 = 0.16 to 0.41). In contrast, distinct seasonal fluctuations in plasma LH occurred in ovariectomised cows, with increases in spring and winter. Environmental cues induced greater fluctuations in plasma LH in ovariectomised cows at a greater stocking rate (LORC 3rd-5th; r2 = 0.71 to 0.72) compared with ovariectomised cows at a lesser stocking rate (LORC 3rd-5th; r2 = 0.53 to 0.58). The findings demonstrated that marked seasonal changes in reproductive activity of the hypothalamic pituitary axis can occur in cows with B. indicus and B. taurus genotypes in a subtropical environment; however, changes in plasma concentrations of LH are only apparent in ovariectomised cows. Concentrations of plasma LH in ovariectomised Brahman and Hereford-Shorthorn cows increased during winter, when pasture availability was limiting and cyclic luteal function ceased in four of five Hereford-Shorthorn cows.

A statistical evaluation of animal and nutritional factors influencing concentrations of milk urea nitrogen

Data from 35 trials with 482 lactating cows fed 106 diets were used to study the effects of animal and dietary factors on the relationship between milk and blood urea N and the value of milk urea N in the assessment of protein status. In two trials, urea N in whole blood and in blood plasma were closely related (r2 = 0.952); the slope was not significantly different from 1.0, and the intercept was not significantly different from 0. Regression of milk urea N on blood urea N with a mixed effects model using all 2231 observations yielded the equation: milk urea N (milligrams of N per deciliter) = 0.620 x blood urea N (milligrams of N per deciliter) + 4.75 (r2 = 0.842); this model accounted for a significant interaction of cow and blood urea N. Single factors that yielded significant regressions on milk urea N with the mixed effects models were dietary crude protein (CP) (percentage of dry matter; r2 = 0.839), dietary CP per megacalorie of net energy for lactation (NEL) (r2 = 0.833), excess N intake (r2 = 0.772), N efficiency (r2 = 0.626), and ruminal NH3 (r2 = 0.574). When all factors were analyzed at once, 12 were significant in a mixed effects model. Blood urea N, body weight, yield of fat-corrected milk, dietary CP content, excess N intake, dry matter intake, and days in milk were positively related to milk urea N, and parity, milk and fat yield, dietary CP per unit of NEL content, and NEL intake were negatively related to milk urea N. In one trial, the mean urea concentration was 35 times greater in urine than in milk; lower proportions of total urea excretion in milk were observed in the a.m. sampling (1.8%) than in the p.m. sampling (3.3%). Measuring urea N in a composite milk sample from the whole day substantially improved reliability of data. The number of cows fed a specific diet that must be sampled to determine mean milk urea N within 95% confidence intervals with half widths of 1.0 and 2.0 mg of N/dl was estimated to be 16 and 4, respectively.

Dietary excesses of urea influence the viability and metabolism of preimplantation sheep embryos and may affect fetal growth among survivors

In the first of two experiments investigating the effect of dietary urea on the survival and metabolism of ovine embryos, 30 Border Leicester x Scottish Blackface ewes received a maintenance diet (milled hay, molasses, minerals, vitamins) with no urea (control, C; n = 10) or with added urea at 15 g (low urea, LU; n = 10) or 30 g (high urea, HU; n = 10) kg-1 feed for a 12 week period. The degraded nitrogen (N) status relative to estimated rumen microbial N requirements was -2, +9 and +20 g per day, respectively. One week after allocation to diets, progesterone priming (12 days) commenced. Ewes received 800 IU of equine chorionic gonadotrophin at progesterone withdrawal, were inseminated 52 h later (Day 0) and embryos were collected from five ewes per group at Day 4 and from five ewes at Day 11. If available, one embryo was returned to each ewe; the rest were cultured in vitro. There was no effect of treatment on progesterone, luteinizing hormone (LH), or time of oestrus onset C, LU and HU plasma urea (P < 0.001) and ammonia levels (C vs. HU, P < 0.01; LU vs. HU, P < 0.05) differed. Day 4 HU embryos were retarded relative to C and LU embryos. After 3 days of culture, 70%, 66% and 0% of C, LU and HU embryos, respectively, were viable. Mid-term pregnancy rates following transfer were 63%, 43% and 33%. Only one HU lamb (male) was born following embryo transfer, its birthweight (10.1 kg) exceeded that of its C (n = 3; 7.0, 7.0, 7.5 kg) and LU (n = 2; 7.3, 8.2 kg) counterparts (P < 0.025). In the second experiment, C2 (2.5 g urea kg-1; n = 5) and HU2 (30 g kg-1; n = 7) diets which provided similar intakes of degraded N relative to microbial requirements as those for C and HU ewes in Experiment 1 were fed to Border Leicester x Scottish Blackface ewes superovulated with 16 mg of porcine follicle-stimulating hormone. Urea and ammonia levels in utero-oviductal samples were elevated in HU2 ewes (P < 0.05). At collection (Day 3), HU2 embryos used more glucose (P < 0.01) and, following culture, some exhibited up to a 2.8-fold increase in metabolism. In conclusion, excess rumen degradable N in ewe diets elevates urea and ammonia in plasma and in utero, with an associated increase in embryo mortality. Nevertheless, metabolism appears to be up-regulated in some embryos and, among those that survive, fetal growth appears to be enhanced.

Effects of dietary crude protein, breed, parity, and health status on the fertility of dairy cows

A study was conducted to determine the impact of dietary CP (13% vs. 20%), parity (first vs. second lactation or later), and breed (Holstein vs. Jersey) on the reproductive efficiency of dairy cows. Sixty-four cows were blocked by parity and breed and assigned to one of two treatments. Cows were removed from treatments on d 100 or 120 depending on pregnancy status. Cows were categorized by health status based on the occurrence of postparturient disorders. Plasma urea N concentrations were influenced by diet (8.6 vs. 21 mg/dl, 13 and 20% CP, respectively), parity, and breed. Reproductive indices were not influenced by diet except that days to first estimated ovulation increased for cows fed the 20% CP diet when health status was added to the model. Days to first observed estrus, first AI service, and cumulative pregnancy rate were affected by health status. Regression analysis for survival showed an interaction of diet and health status for days open. High CP diets tended to increase days open when cows had major health problems; otherwise, a high CP diet decreased days open. The implementation of a strict reproductive management program allowed high reproductive efficiency goals to be achieved regardless of plasma urea N concentrations.

Effect of the degradation of organic matter and crude protein on ruminal fermentation in dairy cows

The potential of the dacron bag technique to assess fluctuations in ruminal metabolites was studied using 40 Israeli-Friesian dairy cows assigned to an experiment with a 2 x 2 factorial design. Diets contained a low (62%) or high (65%) percentage of ruminally degradable CP and a low (55%) or high (59%) percentage of ruminally degradable OM. Metabolites were monitored before feeding and at 3 and 6 h postfeeding. Before feeding, total VFA and propionate were higher, and acetate and pH were lower, in diets containing a high percentage of ruminally degradable OM than in diets containing a low percentage of degradable OM. By 3 H postfeeding, acetate, butyrate and pH were lower, and propionate was higher, in the diets containing a high percentage or ruminally degradable OM than in the diets containing a low percentage of ruminally degradable OM. By 6 h postfeeding, propionate was higher, and acetate was lower, in diets containing a high percentage of ruminally degradable OM than in diets containing a low percentage of ruminally degradable OM. In the diets with a high percentage of ruminally degradable OM, before feeding and by 3 h postfeeding, ammonia concentrations were higher and lower, respectively, relative to the diets containing a low percentage of degradable OM. Milk yield and composition and DMI were similar among treatments. The correlation was good between the degradability data obtained by the dacron bag technique and the meal-induced variations in ruminal metabolites. The lack of a positive yield response to controlled fluctuations in ruminal metabolites may be related to surplus CP intake.

Review: dairy cattle reproductive physiology research and management--past progress and future prospects

Artificial insemination developed as the solution for two important problems in the dairy cattle industry during the past 50 yr: 1) the need for genetic improvement and 2) the elimination of costly venereal diseases. Cooperation among researchers, extension workers, veterinarians, dairy producers, and emerging AI organizations in pooling their expertise, was instrumental to the remarkably rapid development of AI. The cooperation of universities, government, and producers to fund teams of reproductive specialists to collaborate and transfer findings quickly to potential users was a major component of this successful venture. Money invested in these experiments was estimated to have returned about $100 for each $1 invested. Successful freezing of sperm led to the development of the field of cryobiology, and AI paved the way for embryo transfer. The development of ultrasound equipment; various types of rapid hormone assays; prostaglandins, progestogens, and GnRH; and computerization made possible various alternative management plans for controlling reproduction. Multidisciplinary, multigeographical teams that gather basic needed information have the potential for making excellent progress. As herd size increases, new programs for efficient reproductive management and for identifying needed research through computer modeling are a must. Sexed embryos from elite cows and bulls will be used selectively. When embryonic stem cell technology becomes practical, it will revolutionize cattle breeding.

Responses in urea and true protein of milk to different protein feeding schemes for dairy cows

Four multiparous Holstein cows were used in a 4 x 4 Latin square to investigate the effects of protein concentration, degradability, and quality on plasma urea concentration and milk N constituents. Diets varied in the amount and proportion of RDP and RUP relative to NRC requirements: diet 1, excessive RDP, deficient RUP; diets 2 and 3, balanced for RDP and RUP; and diet 4, excessive RDP, balanced for RUP. Diet 3 was formulated for optimal AA balance as predicted by the Cornell Net Carbohydrate and Protein System. Diets contained 34% corn silage, 19% alfalfa haylage, and 49% concentrate (DM basis). Concentrates varied in amounts of urea and soybean, corn gluten, and fish and blood meals. Concentrations of urea N and NPN in milk varied among diets: diet 1, 19 and 34 mg/dl; diet 2, 16 and 31 mg/dl; diet 3, 15 and 30 mg/dl; and diet 4, 23 and 39 mg/dl, respectively. Increases in NPN concentration were attributed to increases in the urea fraction of NPN. Intake of RUP and AA balance influenced milk true protein content; diet 1, 2.89%; diet 2, 2.90%; diet 3, 3.01%; and diet 4, 2.95%. the proportions of true protein and urea in milk are influenced by CP concentration, protein type, and protein quality.

Effect of supplemental fish meal on the lactation and reproductive performance of dairy cows

The influence of high protein diets (21% CP, DM basis), containing varied percentages of RUP, on lactation performance and fertility was evaluated. Sixty-two Holstein cows (65% multiparous) were blocked by age and randomly assigned to a 2 x 2 x 2 factorial design from d 12 to 125 postpartum. Factor 1 was 0 or 3.5% fish meal diet, factor 2 was location (Calan door versus free stall), and factor 3 was parity (first versus second or later). The soybean meal diet consisted of alfalfa hay, corn silage, barley, and soybean meal. The fish meal diet contained 3.5% fish meal (ruminant grade menhaden) that replaced a portion of the soybean meal. Cows fed the fish meal diet (40% RUP) had DMI, BW, and body condition similar to those of cows offered the soybean meal diet (34% RUP). Cows receiving the fish meal supplement tended to have higher milk protein production throughout the trial, higher milk production during the first 6 wk, and significantly lower ruminal ammonia concentrations than cows receiving the soybean meal diet. Differences in reproductive performance were not significant except for a diet by housing location interaction for conception rates from first AI.

Mammary and renal excretion of purine metabolites in relation to energy intake and milk yield in dairy cows

This investigation was conducted to quantify the purine excretion of cows and to evaluate milk allantoin as an estimator of synthesis of microbial protein in the rumen. Concentrations in plasma of allantoin and uric acid and their excretion in urine and milk were determined using 16 Holstein cows at 21 to 315 d of lactation and yielding 8 to 36 kg/d of milk. The mean daily intake of a 55:45 roughage and concentrate diet was 16.3 kg of DM with 106 MJ of NEL and 13.8 to 16.0% CP. Total urine was collected over 24 h, plasma samples collected four times, and milk samples collected twice on the same day. Analyses of purine metabolites were performed with HPLC. On average, excretion of allantoin in urine and milk was about 294 and 4.1 mmol/d for allantoin and 35 and 1.1 mmol/d for uric acid. Excretion of allantoin in milk was correlated to concentration in plasma (r = .84). As milk yield increased, percentage of milk allantoin increased from .6 to 2.4% of total allantoin excretion (r = .85). The correlations of NEL intake with allantoin excretion in milk (r = .80) and in urine (r = .84) were similar. Daily milk yield was correlated with plasma allantoin (r = .78) and milk allantoin excretion (r = .95). We concluded that determination of milk allantoin is a useful, noninvasive method to monitor rumen microbial protein synthesis.

Pregnancy rates in dairy cows following the administration of a GnRH analogue at the time of artificial insemination or at mid-cycle post insemination

Lactating Holstein dairy cows (n=1,533) were allocated to one of three treatment groups, with Group I (n=514) receiving 10 microg of a GnRH analogue (buserelin) at artificial insemination (AI) and Group II (n=503) receiving 10 microg of the same analogue at both the time of AI and at 12 days post AI. Herdmates in Group III (n=516) were inseminated on the same day and served as contemporary AI controls. The trial was conducted on five large dairy farms during the spring and summer months in Saudi Arabia. Pregnancy rates were determined by palpation per rectum between 33 and 50 days following AI. The first service pregnancy rate for the control cows (42.4%) was lower (P<0.05) than that for cows treated with the GnRH analogue at AI (48.8%) or for the combined treatment at AI and at Day 12 post AI (51.5%). No additive effect on the pregnancy rate was noted from the combined analog treatment. The overall increase in pregnancy rate from the analogue treatment at AI resulted from an 11% increase in pregnancy rate in first parity cows over that of contemporary controls (P<0.05) and a 14.7% increase in pregnancy for cows mated at 40 to 59 days post partum and treated with the analogue at AI over that of the corresponding controls (P<0.05). The pregnancy rates from repeat AI (interval<or=30 days) were similar across treatments (45.7% for Group I, 42.9% for Group II and 47.8% for Group III). In this study, the GnRH analogue may have enhanced the ovulation rate and possibly luteal function in cows with reduced hypothalamic GnRH release under environmental stress.

Nutrition and reproduction in dairy cows

Diagnosis of nutritional effects on herd fertility requires a systematic analysis of reproductive records in addition to quantifying the feeding program on the farm. Corroborative laboratory tests for specific deficiencies may be useful.

Relationship of changes in condition score to cow health in Holsteins

The relationship of body condition score with disease occurrence was examined in 561 cows in nine herds. Cows were body condition scored on a five-point scale (1 = thin, 5 = obese) every 2 wk from drying off until 150 d in milk. Cows scored between 3- and 3+ were considered to be in average or good condition. Cows scored less than or equal to 2+ were considered to be underconditioned, whereas those scored greater than or equal to 4- were considered to be overconditioned. Relationships of health and condition score were examined using multiple logistic regression for dichotomous outcomes (e.g., diseased or healthy). Cows that developed dystocia or were culled lost more condition during the dry period than those that did not develop dystocia or were not culled. Cows overconditioned at drying off were more likely to develop cystic ovarian disease and reproductive problems. Cows underconditioned or overconditioned at drying off were more prone to foot problems after calving. Cows overconditioned at 30 d postpartum were more likely to have metritis.

Interactions of nutrition and reproduction

As dairy cattle achieve higher milk production, we are becoming aware of closer relationships between nutrition and fertility. An increasing body of evidence indicates that excessive protein intake in early lactation may be detrimental to postpartum fertility. In addition, protein solubility and degradability are important. Energy restriction in high producing cows may be detrimental to fertility through its effect on hypothalamic and ovarian function. Dairy producers are susceptible to advertisements advocating the addition of specific vitamins and minerals to the rations of high producing cows since inadequate or excessive quantities may be detrimental to fertility. This is particularly true for minerals. However, high producing cows do not usually receive grossly inadequate or excessive dietary sources of vitamins and minerals. Rather, they may be presented with a borderline intake of several vitamins and minerals, which together may be detrimental to fertility. Finally, new methods must be developed to provide a more sensitive indication of fertility. These are some of the research and educational needs of the dairy industry if it is to continue successfully increased milk production. Predictions of herd averages of 16,000 kg milk and individual records exceeding 30,000 kg have been made for the yr 2000.