In two experiments, two liveweight gam treatments were imposed under pastoral grazing conditions on 1 5-month-old Angus heifers from 2 1 days (one oestrous cycle) prior to insemination until day 90 (Experiment 4.2) or day 9 1 (Experiment 4. 1 ) of pregnancy (end of first trimester). The two treatments consisted of moderate maternal l iveweight gain (target 500 g/day) and low maternal liveweight gain (Experiment 4. 1 : target 1 00 g/day; Experiment 4.2: target 0 g/day). Liveweight gain was managed by adjusting the herbage allowance of the groups in response to unfasted live weights that were measured at a maximum of monthly intervals throughout the treatment period. A total' of 376 heifers that had already received one insemination were re-syncrhonised according to the procedure described previously (Chapter 5) to stimulate a second synchronised oestrus. For both experiments, DO was defined as the first day of inseminations at the second synchronised oestrus. Oestrus detection aids (Estrus Alert®, Western Point Incorporated, USA) were appl ied to the heifers on D-2. Heifers (n=58 Experiment 4. 1 , n=60 Experiment 4.2) were inseminated to observed oestrus on DO (Experiment 4. 1 & 4.2) or D 1 (Experiment 4.2) with semen from Angus bulls (Experiment 4. 1 : n=5 ; BeefPac, LIC, Hamilton, New Zealand; Experiment 4.2: n=4). Heifers were excluded from the experiments if they did not show oestrus on DO (Experiment 4. 1 & 4.2) or D 1 (Experiment 4.2), or if they were diagnosed non-pregnant using rectal palpation on D43. Heifers were randomly selected from the remaining heifers for inclusion in the experiments (proportion selected was 1 6/25 for Experiment 4. 1 and 24/36 for Experiment 4.2).
Measurements
Unfasted maternal l ive weights were recorded on 089 (Experiment 4. 1 ) or 088 (Experiment 4.2). At 09 1 (Experiment 4. 1 ) or 090 (Experiment 4.2), the heifers were slaughtered through a commercial processing plant (AFFCO Manawatu, Feilding, New Zealand). Number of heifers was eight in each treatment for Experiment 4. 1 , and nine in the moderate and 15 in the low treatment in Experiment 4.2. At slaughter, maternal l ivers were weighed immediately (Experiment 4. 1 : with gall bladders on; Experiment 4.2: with gall bladders removed) and gravid uteri were recovered and refrigerated overnight. Carcass weights were provided by the processing plant and dressing out
percentage was calculated by dividing hot carcass weight by the l ive weight recorded on D89 (Experiment 4. 1 ) or D88 (Experiment 4.2).
The fol lowing day, excess tissue was trimmed from the uterus before the gravid uterus was weighed intact. The uterus was pierced and drained and the fetus removed. Placentomes were separated and the cotyledons and caruncles were dissected from the fetal and maternal membranes. Weight was recorded of the fluid drained from the uterus and membranes, the fetal membranes (including cotyledons), the empty uterus (including caruncles), the total cotyledons and the total caruncles. Number of cotyledons and number of caruncles were also recorded. Caruncle utilisation was
number of cotyledons .
calculated as x 1 00 . Fetuses were weighed intact and crown-
number of caruncles
rump length, head circumference, thoracic girth and cannon bone length were measured. Fetal heart, liver, lungs and kidneys were dissected out and weighed wet. In Experiment 4.2, wet weight of fetal spleen, thymus, thyroid glands and adrenal gland was also recorded. Fetal body weight (g) per 1 00 kg maternal weight was calculated as
maternal live weight 2 days prior to slaughter
Statistical methods
Data were analysed using the MIXED procedure in the Statistical Analysis System (SAS version 9. 1 , SAS Institute Inc., Cary, NC, USA, 2003). The model for maternal parameters considered the fixed effects of experiment and treatment nested within experiment. Treatments were nested within experiment because the ' moderate' and 'low' treatments varied in the rate of liveweight gain achieved between experiments. Live weight of the heifer two days prior to slaughter was fitted as a covariate for weight of maternal liver to allow comparisons of implied metabolic rate at a constant body weight (Hersom et al. 2004). Initial live weight of the heifer (recorded on D-23) was fitted as a covariate in the models for l ive weight of the heifer two days prior to slaughter, carcass weight of the heifer and dressing out percentage of the heifer. The model for fetal parameters included experiment and treatment nested within experiment as fixed effects, and fetal body weight as a covariate (except in the models for fetal body weight). Sex of fetus as a fixed effect and age of fetus as a covariate were tested and included in the model if they were significant. The model for uterine and placental parameters included the fixed effects of experiment and treatment nested within
Fetal weight at the end of the first trimester of gestation
experiment. Sex of fetus as a fixed effect, age of fetus as a covariate and interactions between sex of fetus and treatment nested within experiment were tested and included in the model if they were significant.
Resu lts
Heifers from the moderate treatment were heavier at slaughter than heifers from the low treatment in both experiments (P<O.OO I ; Table 8. 1 ). Mean initial live weight (D-23) of the heifers that were slaughtered was similar among all treatments. Actual growth rates achieved were 45 1 ± 32 g/day for the moderate group and 1 1 8 ± 32 g/day for the low group in Experiment 4. 1 and 595 ± 30 g/day for the moderate group and 7 ± 23 g/day for the low group in Experiment 4.2. Carcass weight and dressing out percentage were greater (P<O.OO l ) in the moderate than the low group in both experiments. Weight of maternal liver was related (P<O.OO I ) to maternal live weight and was greater (P<O.OO l ) in the moderate than the low treatments in both experiments, even with adjustment for maternal Ii ve weight.
Fetal body weight not affected by treatment, but fetal weight per 100 kg maternal live weight was greater (P<O.OO I ) in the low compared with the moderate treatment. Male fetuses were heavier than female fetuses. Treatment did not affect body dimensions or the weight of any fetal organ. Body dimensions and most fetal organ weights were related to fetal body weight. There were differences (P<O.05) between experiments in crown-rump length, thoracic girth, head circumference, weight of liver and weight of kidneys.
Number of cotyledons, weight of cotyledons, number of caruncles, weight of caruncles, caruncle utilisation, weight of gravid uterus and weight of maternal membranes did not differ between treatments (Table 3). Weight of fetal membranes tended to be greater (P=0.074) in the low compared with the moderate treatments.