The effects of the application of large amounts of nitrogen fertilizer on milk yield, reproductive performance, and energy balance of spring-calved high genetic merit cows : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Sciences in Animal Production at Massey University, Palmerston North, New Zealand
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2005
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Massey University
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Abstract
The intake of crude protein (CP) in amounts considerably above the cow's requirements has been associated by several overseas (OS) reports with reduced cow performance. In New Zealand (NZ), the CP content of pasture during spring is usually above the cow's requirements. During this time, application of nitrogen (N) fertilizer is a common practice which further increases the CP content of pasture. Despite this fact, few experiments have studied the effect of high CP intake on cow performance in NZ, and the existing results are not conclusive. Therefore, the objective of this experiment was to measure the effect of two levels of CP concentration in pasture caused by application of different amounts of N fertilizer, on milk yield, reproductive performance, and energy balance of grazing, spring-calved, high genetic merit cows in NZ.
Two groups, each of 20 high genetic merit cows, were offered a common pasture allowance (46 kg OM/cow/day) over the first 101 days postcalving, but from pasture which contained either a lower (low CP group) or a higher concentration (high CP group) of crude protein. The difference in CP concentration was created by applying either no N fertilizer or 35 kg of N fertilizer after every grazing.
Herbage masses were measured on every paddock weekly, and every day pregrazing and postgrazing, using a rising plate meter. These values were used to calculate pasture growth, and apparent dry matter intake per group. Pasture composition was measured by near infrared spectroscopy.
For each cow, milk yield and composition was measured by weekly herd tests. Reproductive performance was assessed by scanning of follicles and corpora luteal from day one until day 40 postcalving, and from 10 days before the predicted artificial insemination (Al) until 35 days post Al. Progesterone concentrations were measured in milk samples taken every second day.
Liveweight and condition score of each cow were measured weekly, and concentrations of NEFA and IGF1 were measured in blood samples taken
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once every week. Energy balance was calculated as theoretical energy requirements minus estimated energy intake, and feed conversion efficiency was also calculated.
When compared with the low CP paddocks, the high CP paddocks produced more pasture (+1,260 kg OM/ha in 101 days) and it was of higher quality (11.8 vs. 11.4 MJ ME/kg OM; 83.4% vs. 81.2% digestibility; 24.5 vs. 21.6% CP). The apparent daily intakes of OM and ME of the two groups were not significantly different (16.2 and 15.9 kg OM/cow; 195 and 184 MJ ME/ cow). However, the high CP group ate significantly more CP than the low CP group (4.2 vs. 3.5 kg CP/cow/day, and had a higher blood urea concentration (7.8 vs. 5.3 mmol/I).
There were no significant or consistent differences between the two groups in milksolids yield (1.84 vs. 1.91 kg MS/cow/day for the high and low CP groups), reproductive measurements, liveweight, condition score or IGF1 concentrations. However, the high CP group had significantly lower NEFA concentrations than the low CP group (0.45 vs. 0.56 mmol/I). The high CP group had lower apparent feed conversion efficiency than the low CP group, which may have been partially due to the calculated value of 3.2 MJ ME extra required per day to excrete the extra urea from the high CP group.
In conclusion, even though the high CP group had higher blood urea concentrations, no significant differences in cow performance were detected between the two groups. The application of 130 kg N/ha produced an extra 1,264 kg OM/ha, which would have been expected to produce an extra 140 kg MS/ha, although this increase in MS yield may have been reduced slightly (by 9%) by the lower calculated feed conversion efficiency of the high CP group.
Subject for further research include attempts to increase the feed conversion efficiency of animals grazing pastures with high CP content, through the use of sources of extra rumen degradable energy; the effect of diets high in CP on embryo quality and viability; and the ecological impact of using large amounts of N fertilizers.
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Dairy cattle, Feeding and feeds, Reproduction, Milk yield, Nitrogen fertilizers, New Zealand