Low protein diets based on high protein forages for dairy cows

Summary

There is considerable interest in lowering dietary crude protein (CP) concentrations and making greater use of home-grown forages in dairy cow diets due to the high and volatile costs of purchased protein feeds such as soya bean meal, and the legislative requirement to reduce nitrogen (N) and ammonia output from dairy farms. Diets high in CP typically result in a low N use efficiency (NUE), with only around 25% of the N consumed by a cow being captured in milk, with the excess being excreted, particularly in the urine. Excess N in the urine is the major contributor to ammonia release from dairy farms. Some studies have reported that feeding low CP diets decreases milk yield and milk fat and protein content. However, others have shown that dietary protein levels can be lowered to around 140-150 g/kg dietary dry matter (DM) without affecting performance, health or fertility if the diets are formulated to meet the cows metabolisable protein (MP) requirements.

Home-grown forage legumes such as lucerne and red clover are high in CP at approximately 180-200 g/kg DM and can fix N, reducing the need for artificial fertilisers. However, the protein in legume silages is rapidly released in the rumen, which lowers the MP supply, particularly for high yielding dairy cows. Lucerne is the most popular forage legume grown globally, and is more common than grass silage in North America and many areas of Europe. Intake and milk yield are typically higher in lucerne based diets, although high inclusion rates have been shown to reduce milk yield in recent UK based studies.

Key findings

• Dry matter intake was approximately 1.6kg/d less in cows fed the low protein diet and feed conversion efficiency was higher.
• Purchased feed cost savings of between 0.5 to 1ppl can be achieved through lower protein diets.
• Feeding less protein will significantly reduce urinary Nitrogen excretion and improve nitrogen use efficiency.  This will have a major effect on reducing the environmental impact of dairy farming. 

Aims and Objectives

The objectives of this project were to improve the nitrogen use efficiency of dairy cows whilst maintaining performance and reducing purchased feed costs by feeding low protein diets based on high protein, home-grown forage legumes.

The challenge was addressed in 3 controlled studies (2 with red clover and one lucerne silage), and a systematic review and meta-analysis of the literature.

In Study 1, eighteen early lactation Holstein-Friesian dairy cows yielding 45.3 kg/d were fed one of 3 diets based on 50:50 red clover to grass silage (DM basis) and one of 3 dietary CP levels: High (H) – 174 g CP/kg DM; Medium (M) - 165 g CP/kg DM or Low (L) – 153 g CP/kg DM. Each cow received each diet in each of 3 periods of 28 days, with measurements undertaken in the final week of each period. During the sampling week milk yield was recorded daily and samples taken for subsequent analysis. The diets were sampled daily and faecal grab samples collected. Blood samples were collected over two days. Live weight and body condition score were recorded at the start and end of each period.

In Study 2, eighteen Holstein-Friesian dairy cows were fed one of three dietary treatments based on lucerne and maize silage: High protein (H50):  172 g CP/kg DM) with 50:50 lucerne:maize silage

Low protein (L50): 150 g CP/kg DM) with 50:50 lucerne:maize silage;

Low protein (L60):   150 g CP/kg DM) with 60:40 lucerne:maize silage.

 Each cow received each diet in each of 3 periods of 28 days, with measurements undertaken in the final week of each period. During the sampling period milk yield was recorded daily and samples taken for subsequent analysis. The diets were sampled daily and faecal grab samples collected. Blood samples were collected over two days. Live weight and body condition score were recorded at the start and end of each period.

In Study 3, 56 Holstein-Friesian dairy cows were fed one of four diets for 14 weeks. All diets were based on red clover and grass silage (ratio of 1:1, DM basis), and a forage to concentrate ratio of 0.53: C: Control (175 g CP/kg DM); LP: Low protein (150 g CP/kg DM); LS: Low protein (150 g CP/kg DM) with added dietary starch; LM: Low protein (150 g CP/kg DM) with added rumen-protected methionine. The final two treatments were designed to increase microbial protein yield and improve the amino acid profile of the protein. Intake and milk yield were recorded daily, live weight and body condition score fortnightly, and blood samples were collected during weeks 0, 4, 8 and 14. At the end of the 14 week feeding period, 20 cows (5 per treatment) were restrained in stalls to allow a total collection of urine and faeces.

In Study 4 a systematic review and meta-analysis of the published literature was used to investigate the effects of dietary CP concentration on the performance, metabolism and N use efficiency of dairy cows fed forage legume-based rations. A total of 36 studies with 102 treatment means were used.

Key results

• Reducing dietary protein to 150g/kg dry matter in rations based on home- grown forage legumes is achievable and will not affect performance if metaboliasble protein requirements are met and the proportion of forage legume is not too high.
• Feeding very low protein diets (<140g/kg DM) is challenging and is more likely to reduce performance, although nitrogen use efficiency will be further improved.
• Supplementing low protein diets with rumen protected amino acids will increase costs but may not improve performance, although this will depend on the composition of the diet.
• Purchased feed cost savings of between 0.5 to 1ppl can be achieved through lower protein diets.