Urea

Toxicity to reproduction

Administrative data

Endpoint:

reproductive toxicity, other

Type of information:

experimental study

Adequacy of study:

supporting study

Study period:

The study began on 18 August 2003 and finished on 27 November 2003.

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Data source

Reference

Reference Type:

publication

Title:

Effects of application in spring of urea fertiliser on aspects of reproductive performance of pasture-fed dairy cows.

Author:

Ordóñez, A; Parkinson, TJ; Matthew, C; Holmes, CW; Miller, RD; Lopez-Villalobos, N; Burke, J and Brookes I

Year:

2007

Bibliographic source:

New Zealand Veterinary Journal 55(2):69–76

Materials and methods

GLP compliance:

not specified

Test material

Test material information

Constituent 1

Reference substance name:

Urea

EC Number:

200-315-5

EC Name:

Urea

Cas Number:

57-13-6

Molecular formula:

CH4N2O

IUPAC Name:

urea

Details on test material:

It is assumed that the purity is 100%

Test animals

Species:

other: spring-calved dairy cow

Strain:

other: Holstein-Friesian

Administration / exposure

Details on exposure:

The High-N group was grazed on paddocks which received urea fertiliser from 4 weeks before the beginning of the study until its conclusion. Urea was applied at intervals of 4–6 weeks, 1–3 days after paddocks had been grazed. The amount of fertiliser applied varied during the experiment, to maintain a significant difference (p=0.05) in CP content of pastures between treatments. The Control group was grazed on similar paddocks, which did not receive any application of urea during the same period. Paddocks allocated to High-N or Control groups were paired, in a chequerboard layout, within the same block of land on the Massey University No. 4 Dairy Unit (Palmerston North, New Zealand) and all were on a Pallic Tokomaru silt loam. Pasture consisted of a perennial ryegrass (Lolium perenne, Bronsyn AR1 cultivar) and white clover (Trifolium repens, Sustain and Aran cultivars) mix that contained similar proportions of the two species in each paddock. To achieve equal allowances of pasture, despite the expected lower growth rates of pastures that were not fertilised with urea, the High-N group was allocated 7.57 ha (four paddocks) and the Control group 9.68 ha (five paddocks). Cows were placed into the paddocks 4 days after calving.

Each time a new paddock was to be grazed, it was divided into breaks, the area of which was calculated according to the mass of herbage present. A new break of a paddock was offered for each group every day. Groups remained in a paddock until all the breaks were grazed, unless the mass of herbage increased to a level that compromised pasture quality. Some High-N paddocks were grazed by extra cows that were not part of the experiment, to prevent excessive accumulation of pasture and any associated decrease in pasture quality. Urinary and faecal nitrogen inputs from these cows were not taken into consideration. The pasture allowance for both groups was similar and generous (about 40 kg DM/cow/day), to achieve maximum intakes. The mass of herbage after grazing was maintained at around 2,000 kg DM/ha, to ensure that cows could achieve adequate bite sizes and daily DMI. The mass of herbage was assessed using a rising plate meter, and its composition was measured using near infrared analysis (AgResearch, Palmerston North, NZ), as previously described. No supplementary feeds were used after calving.

Details on mating procedure:

inseminations

No. of animals per sex per dose:

20 High-N and 20 control animals

Control animals:

other: . The Control group was grazed on similar paddocks, which did not receive any application of urea during the same period.

Examinations

Parental animals: Observations and examinations:

Concentrations of urea in serum and progesterone in milk were measured. BODY WEIGHT: Yes
- Time schedule for examinations: Cows were weighed and condition-scored weekly.

OTHER:
At the same time (weekly weighing), blood samples (5 mL, no anticoagulant) were collected by coccygeal venepuncture. After clotting for 1 h at ambient temperature, serum was separated by centrifugation at 1,000g for 15 min, and stored at –20°C until assayed for urea concentrations, using standard colourimetric methods (Roche/Hitachi 91; Roche Diagnostics, Basel, Switzerland). The sensitivity of the urea assay was 1.8 mmol/L and the inter- and intra-assay coefficients of variation were 3.4% and 0.8%, respectively.

Oestrous cyclicity (parental animals):

Ovarian follicular and luteal dynamics were determined using ultrasonography. Oestrous behaviour and the number, time and outcome of inseminations were also recorded.

Statistics:

Statistical analyses addressed the null hypothesis that there was no difference between the two groups of cows. It should be kept in mind when interpreting results that since the experimental design did not include repetition of blocks, there is a possibility that paddock factors other than urea fertiliser could have influenced the mean values for the two groups.

Progesterone concentrations and ovarian follicle data were subjected to analysis of variance with respect to treatment group and time, using Production Worth as a covariate, in a repeated measures model in which individual animals were nested within groups, using the PROC MIXED of SAS (SAS Institute Inc, Cary NC, USA, 2001). Categorical outcomes were analysed using generalised linear models with a binomial error distribution and logit link, whilst continuous data, such as calving to conception interval, were analysed using generalised linear models with a Poisson error distribution. There were no censored data in the latter categories, i.e. all cows displayed a first oestrus, were inseminated for the first time and a calving to conception interval could be calculated. These analyses were undertaken using Genstat 5 v4.1 (Numerical Algorithms Group, Oxford, UK).

Results and discussion

Results: P0 (first parental generation)

Details on results (P0)

Intervals between calving and first oestrus, first insemination and conception, the time of first emergence of a dominant follicle, milk progesterone concentration, and the diameter of the corpus luteum (CL) in the first luteal phase did not differ significantly between groups. The interval from calving to first ovulation tended (p=0.10) to be lower and the diameter of the dominant follicle of the oestrous cycle at which cows conceived was greater (p=0.02) in Control than High-N cows.
CONCLUSIONS: The use of large amounts of urea fertiliser during spring and the consequent increases in concentrations of CP in pasture and urea in serum did not negatively affect any of the parameters of reproductive performance of pasture-fed dairy cows that were assessed in this study.

Any other information on results incl. tables

Mean CP and metabolisable energy contents were higher (p<0.001), and acid and neutral detergent fibre contents were lower (p<0.05) in pastures grazed by the High-N group compared with the Control group, whereas soluble sugar contents were similar. However, except for CP content, the numerical values of these differences were small. Serum urea concentrations were higher (p<0.05) in High-N than Control cows in the second week of the study, and remained higher thereafter. There was no significant effect of treatment on bodyweight or change in bodyweight over time (p>0.40), although cows in the High-N group tended to have lower bodyweights than Control animals during Weeks 6–9 of the study.

 

The effect of treatment on the interval between calving and first ovulation approached, but did not reach, statistical significance (High-N=25.8 (SE 2.5); Control=31.9 (SE 2.7); p=0.10). There was no significant difference between groups in the interval between emergence and ovulation of the first follicle that ovulated, the maximum pre-ovulatory diameter of this follicle, or the interval between emergence and ovulation of the follicle in the oestrous cycle in which conception occurred. However, the maximum pre-ovulatory diameter of this follicle was greater in Control than in High-N cows (p=0.02). The numbers of luteal phases of short (≤10 days) or normal (>10 days) length during the study were identical for the two groups of cows, and the total area under the curve of progesterone concentrations during luteal phases of >10 days’ duration was not significantly different (p=0.3). No luteal phases exceeded 18 days in length.

 

Neither calving to first insemination interval, calving to conception interval, nor conception rate to first insemination was affected by treatment, although calving to first oestrus interval tended to be lower in High-N than in Control cows (p=0.06). Cumulative frequency graphs of these effects showed that the main cause of the difference between treatments was a later onset of oestrus in Control cows that had not cycled before Day 60 postpartum.

Applicant's summary and conclusion

Conclusions:

The use of large amounts of urea fertiliser during spring and the consequent increases in concentrations of crude protein (CP) in pasture and urea in serum did not negatively affect any of the parameters of reproductive performance of pasture-fed dairy cows that were assessed in this study.

Executive summary:

AIMS: To assess if raising concentrations of crude protein (CP) in pasture in spring by the frequent application of urea fertiliser would affect ovarian follicular dynamics, luteal function, onset of oestrus and reproductive performance of dairy cows under farming conditions in New Zealand.

 

METHODS: Spring-calved dairy cows were grazed for 101 days in paddocks that were either not fertilised (Control; n=20) during the course of the study, or were fertilised with 40–50 kg nitrogen (N)/ha every 4–6 weeks (High-N; n=20). Similar generous pasture allowances were offered to both groups. Concentrations of CP in pasture, urea in serum and progesterone in milk were measured. Ovarian follicular and luteal dynamics were determined using ultrasonography. Oestrous behaviour and the number, time and outcome of inseminations were also recorded.

 

RESULTS: Mean concentrations of CP in pasture and urea in serum was higher in the High-N than the Control group (25.2 vs 21.6 and 8.3 vs 5.4 mmol/L for CP and urea, respectively; p<0.001). Intervals between calving and first oestrus, first insemination and conception, the time of first emergence of a dominant follicle, milk progesterone concentration, and the diameter of the corpus luteum (CL) in the first luteal phase did not differ significantly between groups. The interval from calving to first ovulation tended (p=0.10) to be lower and the diameter of the dominant follicle of the oestrous cycle at which cows conceived was greater (p=0.02) in Control than High-N cows.

 

CONCLUSIONS: The use of large amounts of urea fertiliser during spring and the consequent increases in concentrations of CP in pasture and urea in serum did not negatively affect any of the parameters of reproductive performance of pasture-fed dairy cows that were assessed in this study.