Effect of differences in live weight on feed requirements of pregnant non-lactating grazing dairy cows : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Animal Science at Massey University, Palmerston North, New Zealand

Abstract

The effect of differences in live weight (LW) on feed requirements of pregnant non-lactating cows was assessed during a 41-day grazing experiment. Thirty eight dry pregnant Friesian and Jersey cows (28 Friesian cows differing in live weight and 10 Jerseys) at similar stages of pregnancy (range 190 to 230 days pregnant) and averaging 5.8 years of age were used. The cows were grouped according to their initial LW in three size-groups, i.e. Big Friesians (BF; n=14, LW = 526 kg), Small Friesians (SF; n=14, LW = 415 kg) and Jerseys (J; n=10, LW = 362 kg). Within each size-group the cows were randomly allocated to one of two levels of daily herbage dry matter (DM) allowance (HA), calculated to meet either maintenance and pregnancy (i.e. HA of 7.7 to 11.0 kg DM/cow/day), or the gain of 1 kg of maternal live weight above maintenance and pregnancy (i.e. HA of 17.1 to 22.5 kg DM/cow/day). The cows provided individual records of their daily liveweight gain (LWG, kg/cow), total liveweight gain (ΔLW) and total condition score change (ΔCS) achieved during the 41-day experimental period. Group average herbage dry matter intake (DMI) and herbage DM allowance were calculated for each treatment group from herbage mass (HM) assessed by cutting-washing-drying and weighing, and by means of two calibration equations, one for each level of feeding, relating HM to the average of 30 plate meter readings (PMR) taken every day before and after grazing. These two calibration equations were: (1) for the ad libitum level of feeding: HM (kg DM/ha) = 764.0 (s.e. 212.0) + 158.0 (s.e. 12.7) * PMR (r = 0.98; CV = 24%; r.s.d. = 548 kg DM), and (2) for the maintenance fed cows: HM (kg DM/ha) = 171.0 (s.e, 3.5) * PMR (r = 0.98; CV = 21.6%; r.s.d. = 442 kg DM). The energy content of the herbage (MJ ME/kg DM) apparently grazed by the cows and their metabolizable energy intake (MEI) were calculated from the in vitro digestibility analyses of pasture samples plucked randomly from each of the grazing areas. Least squares means were calculated for group average herbage dry matter intake (DMI), herbage DM allowance (HA), metabolizable energy intake (MEI), and for the variables derived from the animals' performance (ΔLW, LWG, ΔCS) and differences between levels of feeding and size-groups were tested for significance using analysis of variance. Differences in average live weight between the three size-groups were highly significant (P<0.001) throughout the experimental period (i.e. BF = 552 kg; SF = 442 kg; J = 377 kg). Heavier cows had: (1) significantly higher daily herbage DM allowances (BF, 16.7; SF, 14.4; J, 12.4 kg/cow/day); (2) higher daily DMI (BF, 10.2; SF, 8.6; J, 7.5 kg/cow/day); (3) higher MEI (BF, 117; SF, 100; J, 87 MJ/cow/day), and (4) lower stocking densities (BF, 240; SF, 262; J, 305 cows/ha/24 hours). However, when HA, DMI and MEI where expressed on a metabolic weight basis, none of these variables were significantly different between the three size-groups. From the least squares means of LWG, ΔCS, DMI and MEI calculated for each treatment group, feed requirements for zero ΔCS or maintenance (i.e. MEm) and feed requirements for ΔCS were calculated by means of linear regression analyses. The MEm calculated pooling the three size-groups was 0.648 MJ ME/LW0.75/day for zero ΔCS; and an average intake of 167 kg DM or 1986 MJ ME/cow above maintenance was required for the gain of one condition score unit/cow during the 41 days of experimental period, which was equivalent to a total liveweight change of 52.7 kg/cow. From these estimates it was calculated that cows heavier by 100 kg required an extra intake for maintenance of 10.5 MJ ME/cow/day or about 0.95 kg herbage dry matter intake/cow/day. The results of the present experiment were used to assess the effect of farming large-size cattle on the productive efficiency of pasture-based dairy systems.